engine: Prevent vnic live update for if network not on host

The patch blocks updating a plugged vnic on a running vm to
a network which doesn't exist on the host.

It also fixes a bug of setting an empty vnic profile while
updating a plugged vnic.

Change-Id: I0c9d8f2202efa2e45a2144bbdb70d374bdb9ff6f
Bug-Url: https://bugzilla.redhat.com/1017602
Signed-off-by: Moti Asayag <masayag@redhat.com>