webadmin: Disabled VNIC properties for external networks

Disabled ability to hot update link state and change network while a
VM is running, and to set port mirroring, for external networks.

Change-Id: Ib598d97d76dbfe647a0f71e9269d0a11f68ceaf0
Signed-off-by: Lior Vernia <lvernia@redhat.com>