virttest: Add bridge add and delete function to class Bridge

Add two functions to class Bridge to handle bridge add and delete.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest: Add a function to list nic interface in host

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1634 from ypu/utils_net

Add some functions to utils_net