Must unfreeze while stopping container

Must unfreeze while stopping container

As pointed out by Dan Smith, when a container is being stopped, it must
also be unfrozen after posting the SIGKILL. Otherwise if the container
is frozen when the SIGKILL is posted, the SIGKILL will remain pending
and the lxc-stop command will block until lxc-unfreeze is explicitly
called).

(lxc-stop waits for the container to exit and close the socket but since
the container is frozen, lxc-stop will block).

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Acked-by: Matt Helsley <matthltc@us.ibm.com>
Acked-by: Dan Smith <danms@us.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>