engine: Allowed vNIC activation when VM status is ImageLocked

Since adding VM interfaces was enabled in the add VM dialog, vNIC
activation might also happen when the VM is in status ImageLocked (and
not just Up or Down).

Change-Id: I04d8d6478e0593f89d2c221b2222b6764fd3cbc5
Bug-Url: https://bugzilla.redhat.com/1028755
Signed-off-by: Lior Vernia <lvernia@redhat.com>