conflict of loopback and ip6_tunnel devices mac addresses

[jtluka]

Various devices can have 00:00:00:00:00:00 mac address. I've captured a debugging output that shows

[{'hwaddr': '00:00:00:00:00:00', 'name': 'lo'}, {'hwaddr': '00:00:00:00:00:00', 'name': 'ip6tnl0'}, {'hwaddr': '00:00:00:00:00:00', 'name': 'ip6_vti0'}]

That results in traceback from controller,

    Traceback (most recent call last):
      File "./lnst-ctl", line 124, in get_recipe_result
        res = exec_action(action, nettestctl)
      File "./lnst-ctl", line 81, in exec_action
        return nettestctl.run_recipe()
      File "/root/lnst/lnst/Controller/NetTestController.py", line 627, in run_recipe
        self._prepare_network()
      File "/root/lnst/lnst/Controller/NetTestController.py", line 204, in _prepare_network
        iface.configure()
      File "/root/lnst/lnst/Controller/Machine.py", line 814, in configure
        raise MachineError(msg)
    MachineError: More than one device with hwaddr 00:00:00:00:00:00 found on machine site1-router

I'm wondering whether we should match by device name insted of mac address here. LNST expects that there is 'lo' device anyway. The only bit missing is map_if_by_devname().