In the past, lxc-cmd-stop would wait until the command pipe was closed
before returning, ensuring that the container monitor had exited.
Now that we accept the actual success return value, lxcapi_stop can
return success before the monitor has fully exited.

So explicitly wait for the container to stop, when lxc-cmd-stop returned
success.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>