virttest: Add support for set up private openvswitch bridge in test machine

Add a class in test_setup.py to support set up private openvswitch bridge
in the host for our tests. Also fixed a problem that in clean up the
private bridge will not init with params, lead to the private bridge name will
always be the default value.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>