autotest/virt-test — pull request #1621 by will-Do (2014-04-23 06:50:15+00:00)
Title: Fix two  interface related problems

1. ```function bring_down_ifname(ifname)``` raise a ```TAPBringUpError(ifname)``` error, which maybe copied from ```function bring_up_ifname(ifname)```, so define a new exception ```TAPBringDownError``` for the function.
2. 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.