webadmin: Disable port mirroring for external networks

Operation was blocked by backend, but not mirrored (pun intended) in
the GUI.

Change-Id: Idafb770e2a96ed8a85e6ef797a7e1a03998e46af
Bug-Url: https://bugzilla.redhat.com/1058778
Signed-off-by: Lior Vernia <lvernia@redhat.com>