libqos: use generic qtest_shutdown()

libqos: use generic qtest_shutdown()

Machine specific shutdown function can be registered by
the machine specific qtest_XXX_boot() if needed.

So we will not have to test twice the architecture (on boot and on
shutdown) if the test can be run on several architectures.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>