engine: Allow network update if vnics are unplugged

If a network is configured on running VMs, but no active
nic is using it, the update should be permitted.

Change-Id: I0bdbc159edb95b530b1c0b04b81589495a2c1123
Bug-Url: https://bugzilla.redhat.com/1059741
Signed-off-by: Moti Asayag <masayag@redhat.com>