Handle missing libvirt vnic targets!

Updates the libvirt inspector so that our vnic inspection
can handle cases where the interface/target is missing.
This can occur in some (usually transient) cases where
Nova is pausing/migrating/etc an instance.

Previously Ceilometer would functionally work fine but
a rather long stack trace would get logged to the log file.

Fixes LP Bug #1221784

Change-Id: I7113c6e4bb4af622d517915d13147f5912bad126
Merge "Handle missing libvirt vnic targets!"