Delete vrouter reference on VM deletion

For source-nat service instance, virtual machines composed the service
instance have a back ref on a vrouter. Delete the reference on that
vrouter before deleting the virtual machine.

Change-Id: I697583dbf54662b98f50648ec506123424a6c3ab
Signed-off-by: Édouard Thuleau <edouard.thuleau@cloudwatt.com>
Merge "Delete vrouter reference on VM deletion"