Xen: Cleanup orphan volume connections on boot failure

If the boot process fails after the VDI creation but before a VBD is
created then the current cleanup methods will not work as they all rely
on lookups via VBD.

Change-Id: Id0d93ee60f75bf319baf7859b220ca325175128a
Closes-bug: #1329941
Merge "Xen: Cleanup orphan volume connections on boot failure"