I cannot speak to oVirt in particular, but I run a Elastix PBX on my
Xen server at home (soon to be replaced by oVirt). It is very lightly
used (just my wife and I) but I have had no problems with it.
The company I work for runs our PBX (trixbox) off a VMware cluster in
our colo. It consists of about 4 servers (hosts) with about 16
running VMS. We just recently switched to full VoIP, but prior to
that we were using a PRI card that was accessible over IP (similar to
using something like a SPA-3000 for connecting one's analog home line
via Asterisk/Elastix -- which I currently do on my home setup). We
did not experience any performance issues under this setup (and
currently do not with our all-VoIP config)
I hope this helps.
-Alan