virsh_iface_bridge: add checkpoints for iface-bridge test

add two checkpoints for the test of iface-bridge command
1) The new bridge's IP should keep the same as original interface' IP.
2) If '--no-stp' option isn't specified, the STP should be enabled.
Use 'grep' may not get the exact result, such as:
# brctl show | grep br0
br0                   8000.52540023e956	no		
virbr0		8000.52540023e956	yes		virbr0-nic

BTW, it's better to move this func to virttest/utils_net.py