Don't explode if we fail to unplug VIFs after a failed boot

Libvirt currently attempts to unplug VIFs after destroying an
instance, which can fail if the instance was never built
properly in the first place. This patch makes it ignore errors
when trying to tear down VIFs during that cleanup. Failure
in this case would block the deletion of the instance (which
should never happen) and any real failures aren't something
we could do anything about.

Closes-bug: #1306718

Change-Id: I84a4433c3959b8f14e0259be0aa51da1654006c8
(cherry picked from commit b3225581befe14f21b909e176d0a4583b297e031)