configNetwork: improve assertBridgeClean

Change-Id: I44b28e16d20e993c84d0569db7c661e18d432ce7
Signed-off-by: huntxu <mhuntxu@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/12504
Reviewed-by: Dan Kenigsberg <danken@redhat.com>