snapshot-save very slow in 8.1-rc2
Description of problem:
Before commit 813cd61669 ("migration: Use migration_transferred_bytes() to calculate rate_limit") the above script will take about 1.5 seconds to execute, after the commit, 1 minute 30 seconds. More RAM makes it take longer still.
Steps to reproduce:
1. Execute the script given as the command line above.
Additional information:
Creating the issue here, so it doesn't get lost and is documented.

The following series by @juan.quintela would've avoided the regression, but seems like it never landed: https://lists.nongnu.org/archive/html/qemu-devel/2023-05/msg07971.html

Logs:

Before commit 813cd61669 
```
root@pve8a1 /home/febner/repos/qemu/build # time ~/save-snap.sh
Formatting '/tmp/test.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=1073741824 lazy_refcounts=off refcount_bits=16
{"QMP": {"version": {"qemu": {"micro": 50, "minor": 0, "major": 8}, "package": "v8.0.0-967-g3db9c05a90-dirty"}, "capabilities": ["oob"]}}
VNC server running on ::1:5900
{"return": {}}
{"timestamp": {"seconds": 1691572701, "microseconds": 708660}, "event": "JOB_STATUS_CHANGE", "data": {"status": "created", "id": "save0"}}
{"timestamp": {"seconds": 1691572701, "microseconds": 708731}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "save0"}}
{"return": {}}
{"timestamp": {"seconds": 1691572701, "microseconds": 709239}, "event": "STOP"}
{"timestamp": {"seconds": 1691572702, "microseconds": 939059}, "event": "RESUME"}
{"timestamp": {"seconds": 1691572702, "microseconds": 939565}, "event": "JOB_STATUS_CHANGE", "data": {"status": "waiting", "id": "save0"}}
{"timestamp": {"seconds": 1691572702, "microseconds": 939605}, "event": "JOB_STATUS_CHANGE", "data": {"status": "pending", "id": "save0"}}
{"timestamp": {"seconds": 1691572702, "microseconds": 939638}, "event": "JOB_STATUS_CHANGE", "data": {"status": "concluded", "id": "save0"}}
{"return": {}}
{"timestamp": {"seconds": 1691572702, "microseconds": 939730}, "event": "SHUTDOWN", "data": {"guest": false, "reason": "host-qmp-quit"}}
{"timestamp": {"seconds": 1691572702, "microseconds": 941746}, "event": "JOB_STATUS_CHANGE", "data": {"status": "null", "id": "save0"}}
~/save-snap.sh  1.18s user 0.09s system 85% cpu 1.476 total
```

After commit 813cd61669
```
root@pve8a1 /home/febner/repos/qemu/build # time ~/save-snap.sh
Formatting '/tmp/test.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=1073741824 lazy_refcounts=off refcount_bits=16
{"QMP": {"version": {"qemu": {"micro": 92, "minor": 0, "major": 8}, "package": "v8.1.0-rc2-102-ga8fc5165aa"}, "capabilities": ["oob"]}}
VNC server running on ::1:5900
{"return": {}}
{"timestamp": {"seconds": 1691572864, "microseconds": 944026}, "event": "JOB_STATUS_CHANGE", "data": {"status": "created", "id": "save0"}}
{"timestamp": {"seconds": 1691572864, "microseconds": 944115}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "save0"}}
{"return": {}}
{"timestamp": {"seconds": 1691572864, "microseconds": 944631}, "event": "STOP"}
{"timestamp": {"seconds": 1691572954, "microseconds": 697523}, "event": "RESUME"}
{"timestamp": {"seconds": 1691572954, "microseconds": 697962}, "event": "JOB_STATUS_CHANGE", "data": {"status": "waiting", "id": "save0"}}
{"timestamp": {"seconds": 1691572954, "microseconds": 697996}, "event": "JOB_STATUS_CHANGE", "data": {"status": "pending", "id": "save0"}}
{"timestamp": {"seconds": 1691572954, "microseconds": 698020}, "event": "JOB_STATUS_CHANGE", "data": {"status": "concluded", "id": "save0"}}
{"return": {}}
{"timestamp": {"seconds": 1691572954, "microseconds": 698089}, "event": "SHUTDOWN", "data": {"guest": false, "reason": "host-qmp-quit"}}
{"timestamp": {"seconds": 1691572954, "microseconds": 701263}, "event": "JOB_STATUS_CHANGE", "data": {"status": "null", "id": "save0"}}
~/save-snap.sh  31.81s user 41.69s system 81% cpu 1:30.03 total
```