libvirt: Allow delete to complete when a volume disconnect fails

If an instance which was booted from a volume fails to spawn, the
instance is left in an ERROR state.  If the failure is in the networking
stage Nova will not have attached the volume via the Cinder driver,
and so the volume remains in an available state.  In this state the
volume may be deleted or assigned to another instance.
During the subsequent delete an exception may be thrown from
the cinder driver which prevents the instance from completing the
deletion process leaving the instance stuck in Error and unable to
be deleted.

Volume exceptions should be logged and ignored when the instance
is being deleted.

Closes-Bug #1222979

(cherry picked from commit 506a8f58cf4b8cecf90b647c7deba47da2a4dfec)
Conflicts:
	nova/tests/virt/libvirt/test_libvirt.py

Change-Id: Icb3796b0ddba25cf344953a649b2e762fab6d782
Merge "libvirt: Allow delete to complete when a volume disconnect fails" into stable/havana