Fix "'XenServer' object has no attribute 'device'".

Fixes ZEN-12978.
Merge pull request #7 from zenoss/feature/ZEN-12978

Please review branch feature/ZEN-12978
Release: Version 1.0.2dev -> 1.0.2.
Finished release 1.0.2.
Finished release 1.0.2.
Post Release: Version 1.0.2 -> 1.0.3dev.
Fix SR modeling bug. Deal with bad catalogs.

Since I can't reproduce the issue in ZEN-12824, I don't yet know for
sure if this really fixes the issue. I've given the patch to a user to
try for a while to see if it fixes what is a very reproducible problem
for them.

Fixes ZEN-12824.