Fix InstanceNotFound error in _delete_instance_files

Currently the cleanup_running_deleted_instances peroidic task
failed with InstanceNotFound exception, this is because the
db query is not including the deleted instance while
_delete_instance_files() in libvirt driver.

Closes-bug: #1280140

Change-Id: Ie65ff255ad3c582a71db93b07304a21d4976a193
(cherry picked from d835163759527a651f3c4f2109ca0fdc3e968d37)
Merge "Fix InstanceNotFound error in _delete_instance_files" into stable/havana