I think 5.4 version of python-virtinst added some fixes aswell.. so maybe you need to upgrade to that for F12 installation to work.
Also please try running "xm console" for the guest and see what it does and where it crashes..
If I do xm create -c f12domU.cfg it says "Starting Domain f12domU" and then brings me back to the command prompt on dom0.
Thanks for the replies Pasi, I will try and get some time to bring this host out of production to perform the upgrade.
- Tait