netdev_move_by_index: support wlan

The python lxc-device supported adding wlan devices, so add that
support as well.  Since the python one did not support 'del',
I didn't try adding that support, though it should be trivial to
add.

We should be able to do the wlan adding using netlink, but I
went ahead and used 'iw' as the netlink path looked more
complicated than it does for other nics.  Patches to switch that
over would be very welcome.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>