virttest: Add queues and helper to add_net in qemu_vm.py

Make the add_net support queues and helper options.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest: Make add_net can support invalid paramteres

Make it possible to start the command with invalid parameters to
support some negative tests.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1577 from ypu/qemu_params

Add some params to add_net