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.