Updates lxc_restore_phys_nics_to_netns() to move phys netdevs back to the monitor's network namespace rather than the previously hardcoded PID 1 net ns.

This is to fix instances where LXC is started inside a net ns different from PID 1 and physical devices are moved back to a different net ns when the container is shutdown than the net ns than where the container was started from.
Signed-off-by: Thomas Parrott <thomas.parrott@canonical.com>