Avoid "virsh net-destroy" on inactive bridge

If running "virsh net-destroy" on inactive bridge will get error:
error: Requested operation is not valid: network is not active

Change the logic to a simple&direct one to remove bridge.

Change-Id: Ie0341a1fc09c16c1b9427e847b9dbf9afd25f69e
Closes-Bug: #1256510
Merge "Avoid "virsh net-destroy" on inactive bridge"