Improve libvirt vnic parsing with missing mac!

Improve the libvirt inspector vnic parsing so that it handles
a missing mac address gracefully. Previously a missing MAC
address (if possible) could cause an ugly AttributeError to
get logged to the Ceilometer log file.

Related to LP Bug #1221784.

Change-Id: Ia897b493c9a9493a3ed3d9c3cf54ca29ff944892
Merge "Improve libvirt vnic parsing with missing mac!"