engine: Allowed editing a VNIC when VM image locked

This allows updating VNICs while creating a VM from template.

Change-Id: Id3fa46642beacd2ecc0c6d2ad2bde05c890d8256
Bug-Url: https://bugzilla.redhat.com/996568
Signed-off-by: Lior Vernia <lvernia@redhat.com>