libvirt: log exception info when interface detach failed

The detach_interface method is catching a libvirtError and logging an
error message but it doesn't include the contents of the root
libvirtError, so log that information as well for debugging.

Closes-Bug: #1374140

Change-Id: I042c2956c7393a2ee1ccd026421c8df537aa1a87
Merge "libvirt: log exception info when interface detach failed"