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.
@will-Do 
Thank you for your suggestion.
I have changed the cmd to ""brctl show %s | grep '^%s[[:blank:]]'" % (br_name, br_name)"
and push it to virt-test community as new request.

please help to check it