You would have to set the priority of all vCPUs in each guest, as
well
as emulator threads in each guest (the latter needs very new libvirt).
This does not seem a viable option then with Ubuntu 18.04. And changing XML on all guest
VMs is extra work.
Is there no other way? For example, are all such virtual machines not some daemon or
systemd unit or set of units? Would it be feasible to lower the
priority on all of those? Or is that not easy to do either?
Thanks again,
rdiez