Comment # 28 on bug 1202541 from Dario Faggioli

(In reply to Dario Faggioli from comment #27) 
> but the key point
> here is to stop using host-passthrough for VMs that needs to be migrated.
>
In fact, this is an configuration that is not supported, neither upstream nor
downstream. Simply because it's technically impossible to avoid that VMs will
either crash or malfunction.

If it worked so far, well... Lucky us! :-P

Also, although for sure host-passthrough grants some performance improvement,
I'm quite sure that they're only marginal in this case and for this workload.