engine: allow updating/deleting a network with unplugged vnics only

Allow updating/deleting a network from a host only in case when the
related vnics are unplugged. The case when plugged vnic is unlinked
doesn't allow amending the underlying network.

Change-Id: Ib57fd905811bbbb60ef06bae15d0c42518132596
Signed-off-by: Yevgeny Zaspitsky <yzaspits@redhat.com>