Switch the instance_shutdown rpc to (status, data)

This patch changes the return type from this RPC call to include status
information and renames the backend method to match the RPC call name.

The patch is a little bigger than the reboot one, since this call is
used in more than one place. However, all the points of call have the
same usage pattern, so the patch is trivial.

Reviewed-by: ultrotter