stop cmd callback: unfreeze by path only

stop cmd callback: unfreeze by path only

in particular, regular unfreeze uses the cmd api to request the cgroup
of the container.  If we are already in the lxc-start monitor, we can't
use the cmd api.

(I knew when I started this would be a problem but then as it didn't
reliably crash, I forgot to handle it)

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