interface bug fix: only expose the unused interfaces to the front end

libvirt allows multi bridge networks on a same bridge or nic.
It is not harmful.

But kimchi does not allow this, so if a bridge or a nic is used by a bridge
network, kimchi interfaces API will not expose it to front end.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>