lxc: manually move NICs back to host after container stops

This prevents things like bridges from being destroyed by the kernel.

My hope is that just doing this will be enough to also ensure that
the device will be available to be renamed immediately, so that
we don't need to do a retry loop.

Tested with a dummy device.  renaming dummy0 to dummy5 in container,
then shutting down container, returns dummy0 to the host.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>