Strange throttle-group test results
Description of problem:
I have a question about throttle-group test results.

I did a test to limit IO by applying THROTTLE-GROUP and the expected result is not what I expected

The setup environment looks like this throttle-group to x-iops-total=500, x-bps-total=524288000 and throttling vdb, benchmarked with fio command

```
# mount -t xfs /dev/vdb1 /mnt/disk

# fio --direct=1 --bs=1M --iodepth=128 --rw=read --size=1G --numjobs=1 --runtime=600 --time_based --name=/mnt/disk/fio-file --ioengine=libaio --output=/mnt/disk/read-1M
```

When I test with a --bs value of 1M, I get 500Mib throughput.
![iops_500-1M](/uploads/f63ecbfdb13adc87bd4524f5298a224c/iops_500-1M.png)


When I test with a --bs value of 2m, I don't get 500Mibs but 332Mibs throughput.
```
fio --direct=1 --bs=2M --iodepth=128 --rw=read --size=1G --numjobs=1 --runtime=600 --time_based --name=/mnt/disk/fio-file --ioengine=libaio --output=/mnt/disk/read-2M
```
![iops_500-2M](/uploads/0a384fd9f026943e5e40af1c4b5d6dcd/iops_500-2M.png)


If I set the qemu x-iops-total value to 1500 and the fio --bs value to 2M test again, I get 500Mib throughput.

![iops_1500-2M](/uploads/f31eb8213d034d612e915e355b52a324/iops_1500-2M.png)


To summarize, here is the Test result.

| fio bs | qemu x-iops-total | qemu x-bps-total | Result iops |Result throughput
| ------ | ------ |------ |------ |------ |
| 2M     | 1500   | 524288000 | 250 |  500 |
| **2M** |**500** | **524288000** | **166** |  **332** |
| 1M     | 1500   | 524288000 | 500 |  500 |
| 1M     |  500.  | 524288000 | 500 |  500 |


When the --bs value is 2M and the x-iops-total value is 500, the throughput should be 500, but it is not, so I don't know what the problem is.

If there is anything I missed, please let me know.
Steps to reproduce:
1. Apply throttle-group to vdb and start the VM
2. mount vdb1
3. test fio