utils_net: raise a right error for bring down interface

Signed-off-by: Yanbing Du <ydu@redhat.com>
utils_test.libvirt: Fix a regex

In some case, interface may has no MAC address:
$virsh iface-list
Name                 State      MAC Address
--------------------------------------------
brlo                 inactive
eth0                 active     44:37:e6:4a:42:d1

So fix the regex to get empty MAC address.

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #1621 from will-Do/net_fix

Fix two  interface related problems