It is theoretically possible as qemu supports that. Don't think it's exposed in the UI though. You could probably do something with a script at startup to run virsh to set the throttle parms.
Unless I missed it in the UI!
Specifically, look for the blkiotune section of the manpage. If this was super important to you, I suppose you could fill out an enhancement request for IX and explain your case as to why it should be exposed in the UI.