engine,webadmin: Plugging of an unlinked vnic with an ext net is blocked

Plugged and unlinked VM network interface is not supported for an
external network, thus the action is blocked.

Change-Id: Ia944ca7e0430adc54a6cae6cb65b8a1df4e88d24
Bug-Url: https://bugzilla.redhat.com/1077796
Signed-off-by: Yevgeny Zaspitsky <yzaspits@redhat.com>