engine: Remove vNIC from external network when necessary

Removing the vNIC from the external network only when the vNIC is
removed or the VM gets somehow removed.
This will allow the user of the vNIC to manipulate it on the external
network and not lose his changes when the VM stops.

Change-Id: I027992f52c35add0afd1ffe207b1adbdb2c5c957
Bug-Url: https://bugzilla.redhat.com/1013491
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>