Hi Thomas,
This may be old news at this point, but I had 100% identical behavior to yours when I tried virtx7-44-testing. After looking around a bit I tried "virtx7-44-candidate", in a sibling directory to virtx7-44-testing. It has a more recent date, so I tried with that and it works fine. It resolves the issues listed in your email from 7/7/2015.
I am now looking for a libvirt-daemon-xen package for Centos 7. Any ideas on where that lives?
Thanks, Chuck