Backup with large RBD disk is slow since QEMU 6.2.0 (since commit 0347a8fd)
Description of problem:
Since commit 0347a8fd4c ("block/rbd: implement bdrv_co_block_status"), there is a big slowdown for large RBD images for backup.
Steps to reproduce:
I used the following script
```
root@pve701 ~ # cat rbdbackup.sh
#!/bin/bash
rbd create emptytestA -p rbdkvm --size $2
rbd create emptytestB -p rbdkvm --size $2
$1 \
    -qmp stdio \
    -drive file=rbd:rbdkvm/emptytestA:conf=/etc/pve/ceph.conf:id=admin:keyring=/etc/pve/priv/ceph/rbdkvm.keyring,if=none,id=driveA,format=raw \
    -drive file=rbd:rbdkvm/emptytestB:conf=/etc/pve/ceph.conf:id=admin:keyring=/etc/pve/priv/ceph/rbdkvm.keyring,if=none,id=driveB,format=raw \
<<EOF
{"execute": "qmp_capabilities"}
{"execute": "blockdev-backup",
     "arguments": { "device": "driveA",
                    "sync": "full",
                    "target": "driveB" } }
EOF
rbd -p rbdkvm rm emptytestA
rbd -p rbdkvm rm emptytestB
```
with 200G and 500G images respectively and QEMU binaries built from current master (i.e. 10c2a0c5e7d48e590d945c017b5b8af5b4c89a3c) and from current master with fc176116cdea816ceb8dd969080b2b95f58edbc0, 9e302f64bb407a9bb097b626da97228c2654cfee and 0347a8fd4c3faaedf119be04c197804be40a384b reverted.


Timings:
```
200G master:         92s
200G master+reverts: 57s
500G master:         526s
500G master+reverts: 142s
```

I checked how long a single call to `rbd_diff_iterate2()` in `block/rbd.c` takes, and it seems to take about linearly more time the bigger the image is. But it is also called linearly more often, resulting in about quadratic slowdown overall.
Additional information:
Full commands/output:
```
root@pve701 ~ # ./rbdbackup.sh ./qemu-upstream/10c2a0c5e7d48e590d945c017b5b8af5b4c89a3c/qemu-system-x86_64 200G                 
{"QMP": {"version": {"qemu": {"micro": 50, "minor": 0, "major": 7}, "package": "v7.0.0-981-g10c2a0c5e7"}, "capabilities": ["oob"]}}
VNC server running on 127.0.0.1:5900
{"return": {}}
{"timestamp": {"seconds": 1652695629, "microseconds": 651397}, "event": "JOB_STATUS_CHANGE", "data": {"status": "created", "id": "driveA"}}
{"timestamp": {"seconds": 1652695629, "microseconds": 651447}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "driveA"}}
{"timestamp": {"seconds": 1652695629, "microseconds": 651464}, "event": "JOB_STATUS_CHANGE", "data": {"status": "paused", "id": "driveA"}}
{"timestamp": {"seconds": 1652695629, "microseconds": 651490}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "driveA"}}
{"return": {}}
{"timestamp": {"seconds": 1652695721, "microseconds": 415892}, "event": "JOB_STATUS_CHANGE", "data": {"status": "waiting", "id": "driveA"}}
{"timestamp": {"seconds": 1652695721, "microseconds": 416066}, "event": "JOB_STATUS_CHANGE", "data": {"status": "pending", "id": "driveA"}}
{"timestamp": {"seconds": 1652695721, "microseconds": 416197}, "event": "BLOCK_JOB_COMPLETED", "data": {"device": "driveA", "len": 214748364800, "offset": 214748364800, "speed": 0, "type": "backup"}}
{"timestamp": {"seconds": 1652695721, "microseconds": 416239}, "event": "JOB_STATUS_CHANGE", "data": {"status": "concluded", "id": "driveA"}}
{"timestamp": {"seconds": 1652695721, "microseconds": 416265}, "event": "JOB_STATUS_CHANGE", "data": {"status": "null", "id": "driveA"}}
^Cqemu-system-x86_64: terminating on signal 2
{"timestamp": {"seconds": 1652695727, "microseconds": 145031}, "event": "SHUTDOWN", "data": {"guest": false, "reason": "host-signal"}}
Removing image: 100% complete...done.
Removing image: 100% complete...done.
./rbdbackup.sh  200G  81.15s user 6.31s system 89% cpu 1:38.21 total
root@pve701 ~ # ./rbdbackup.sh ./qemu-upstream/10c2a0c5e7d48e590d945c017b5b8af5b4c89a3c-with-rbd-reverts/qemu-system-x86_64 200G 
{"QMP": {"version": {"qemu": {"micro": 50, "minor": 0, "major": 7}, "package": "v7.0.0-984-g20a19f8eae"}, "capabilities": ["oob"]}}
VNC server running on 127.0.0.1:5900
{"return": {}}
{"timestamp": {"seconds": 1652695737, "microseconds": 444734}, "event": "JOB_STATUS_CHANGE", "data": {"status": "created", "id": "driveA"}}
{"timestamp": {"seconds": 1652695737, "microseconds": 444818}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "driveA"}}
{"timestamp": {"seconds": 1652695737, "microseconds": 444860}, "event": "JOB_STATUS_CHANGE", "data": {"status": "paused", "id": "driveA"}}
{"timestamp": {"seconds": 1652695737, "microseconds": 444885}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "driveA"}}
{"return": {}}
{"timestamp": {"seconds": 1652695794, "microseconds": 437168}, "event": "JOB_STATUS_CHANGE", "data": {"status": "waiting", "id": "driveA"}}
{"timestamp": {"seconds": 1652695794, "microseconds": 437248}, "event": "JOB_STATUS_CHANGE", "data": {"status": "pending", "id": "driveA"}}
{"timestamp": {"seconds": 1652695794, "microseconds": 437341}, "event": "BLOCK_JOB_COMPLETED", "data": {"device": "driveA", "len": 214748364800, "offset": 214748364800, "speed": 0, "type": "backup"}}
{"timestamp": {"seconds": 1652695794, "microseconds": 437368}, "event": "JOB_STATUS_CHANGE", "data": {"status": "concluded", "id": "driveA"}}
{"timestamp": {"seconds": 1652695794, "microseconds": 437381}, "event": "JOB_STATUS_CHANGE", "data": {"status": "null", "id": "driveA"}}
^Cqemu-system-x86_64: terminating on signal 2
{"timestamp": {"seconds": 1652695803, "microseconds": 242148}, "event": "SHUTDOWN", "data": {"guest": false, "reason": "host-signal"}}
Removing image: 100% complete...done.
Removing image: 100% complete...done.
./rbdbackup.sh  200G  40.68s user 111.12s system 228% cpu 1:06.47 total
root@pve701 ~ # ./rbdbackup.sh ./qemu-upstream/10c2a0c5e7d48e590d945c017b5b8af5b4c89a3c/qemu-system-x86_64 500G 
{"QMP": {"version": {"qemu": {"micro": 50, "minor": 0, "major": 7}, "package": "v7.0.0-981-g10c2a0c5e7"}, "capabilities": ["oob"]}}
VNC server running on 127.0.0.1:5900
{"return": {}}
{"timestamp": {"seconds": 1652695970, "microseconds": 663752}, "event": "JOB_STATUS_CHANGE", "data": {"status": "created", "id": "driveA"}}
{"timestamp": {"seconds": 1652695970, "microseconds": 663892}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "driveA"}}
{"timestamp": {"seconds": 1652695970, "microseconds": 663920}, "event": "JOB_STATUS_CHANGE", "data": {"status": "paused", "id": "driveA"}}
{"timestamp": {"seconds": 1652695970, "microseconds": 663980}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "driveA"}}
{"return": {}}
{"timestamp": {"seconds": 1652696496, "microseconds": 556219}, "event": "JOB_STATUS_CHANGE", "data": {"status": "waiting", "id": "driveA"}}
{"timestamp": {"seconds": 1652696496, "microseconds": 556386}, "event": "JOB_STATUS_CHANGE", "data": {"status": "pending", "id": "driveA"}}
{"timestamp": {"seconds": 1652696496, "microseconds": 556497}, "event": "BLOCK_JOB_COMPLETED", "data": {"device": "driveA", "len": 536870912000, "offset": 536870912000, "speed": 0, "type": "backup"}}
{"timestamp": {"seconds": 1652696496, "microseconds": 556536}, "event": "JOB_STATUS_CHANGE", "data": {"status": "concluded", "id": "driveA"}}
{"timestamp": {"seconds": 1652696496, "microseconds": 556555}, "event": "JOB_STATUS_CHANGE", "data": {"status": "null", "id": "driveA"}}
^Cqemu-system-x86_64: terminating on signal 2
{"timestamp": {"seconds": 1652696786, "microseconds": 408273}, "event": "SHUTDOWN", "data": {"guest": false, "reason": "host-signal"}}
Removing image: 100% complete...done.
Removing image: 100% complete...done.
./rbdbackup.sh  500G  453.34s user 28.30s system 58% cpu 13:36.48 total
root@pve701 ~ # ./rbdbackup.sh ./qemu-upstream/10c2a0c5e7d48e590d945c017b5b8af5b4c89a3c-with-rbd-reverts/qemu-system-x86_64 500G
{"QMP": {"version": {"qemu": {"micro": 50, "minor": 0, "major": 7}, "package": "v7.0.0-984-g20a19f8eae"}, "capabilities": ["oob"]}}
VNC server running on 127.0.0.1:5900
{"return": {}}
{"timestamp": {"seconds": 1652695810, "microseconds": 648931}, "event": "JOB_STATUS_CHANGE", "data": {"status": "created", "id": "driveA"}}
{"timestamp": {"seconds": 1652695810, "microseconds": 649012}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "driveA"}}
{"timestamp": {"seconds": 1652695810, "microseconds": 649057}, "event": "JOB_STATUS_CHANGE", "data": {"status": "paused", "id": "driveA"}}
{"timestamp": {"seconds": 1652695810, "microseconds": 649080}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "driveA"}}
{"return": {}}
{"timestamp": {"seconds": 1652695952, "microseconds": 13070}, "event": "JOB_STATUS_CHANGE", "data": {"status": "waiting", "id": "driveA"}}
{"timestamp": {"seconds": 1652695952, "microseconds": 13144}, "event": "JOB_STATUS_CHANGE", "data": {"status": "pending", "id": "driveA"}}
{"timestamp": {"seconds": 1652695952, "microseconds": 13210}, "event": "BLOCK_JOB_COMPLETED", "data": {"device": "driveA", "len": 536870912000, "offset": 536870912000, "speed": 0, "type": "backup"}}
{"timestamp": {"seconds": 1652695952, "microseconds": 13233}, "event": "JOB_STATUS_CHANGE", "data": {"status": "concluded", "id": "driveA"}}
{"timestamp": {"seconds": 1652695952, "microseconds": 13249}, "event": "JOB_STATUS_CHANGE", "data": {"status": "null", "id": "driveA"}}
^Cqemu-system-x86_64: terminating on signal 2
{"timestamp": {"seconds": 1652695955, "microseconds": 692599}, "event": "SHUTDOWN", "data": {"guest": false, "reason": "host-signal"}}
Removing image: 100% complete...done.
Removing image: 100% complete...done.
./rbdbackup.sh  500G  99.49s user 277.78s system 258% cpu 2:25.78 total
```