iotune QoS is using a problematic design requiring excessive use of vdsm (and libvirt) API calls
It can be done more effectively if we take few assumptions (maybe specific to vdsm, but still worth it)

There is no need to query and set policy and values periodically, no one can change the policy without engine and vdsm knowledge.
Changes can be reported as part of VM stats if needed, one field serving as a trigger would be ok. Since VM stats is being queried by mom already there would be no need for any additional calls in stable conditions.

I'm not sure about networking QoS