throttle group limit feature request for discard
Additional information:
- Need to add particular options in [ThrottleGroupProperties](https://qemu-project.gitlab.io/qemu/interop/qemu-qmp-ref.html#object-QMP-block-core.ThrottleGroupProperties) which like this
```txt
x-discard-iops-total
x-discard-iops-total-max
....
```