webadmin: Don't allow to boot from unplugged NIC

PXE boot used to be available for VMs whose VNICs were unplugged, this
is now fixed.

Change-Id: Ic00bb7ef3f463334be6f7605d247f84a2c2adde8
Bug-Url: https://bugzilla.redhat.com/970085
Signed-off-by: Lior Vernia <lvernia@redhat.com>