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

...ine

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.
Shall we output some information if cleanup is not allowed with using TAP?