Comment 23 for bug 1978064

I believe the one reason that adding `cpu_model_extra_flags = -mpx` does not work
is because the models are evaluated before the flags.

So it won't even try to compare the flags on start up before crashing.

This might be a possible way forward if libvirt won't update the cpu_maps
and while we are waiting for "libvirt: Replace compareCPU() with compareHypervisorCPU()"
to be merged
https://review.opendev.org/c/openstack/nova/+/762330/20