xenbus: move xenbus_dev_shutdown() into frontend code...

...and make it static

xenbus_dev_shutdown() is seemingly intended to cause clean shutdown of PV
frontends when a guest is rebooted. Indeed the function waits for a
conpletion which is only set by a call to xenbus_frontend_closed().

This patch removes the shutdown() method from backends and moves
xenbus_dev_shutdown() from xenbus_probe.c into xenbus_probe_frontend.c,
renaming it appropriately and making it static.

NOTE: In the case where the backend is running in a driver domain, the
      toolstack should have already terminated any frontends that may be
      using it (since Xen does not support re-startable PV driver domains)
      so xenbus_dev_shutdown() should never be called.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/xenbus/xenbus_probe_backend.c

@@ -198,7 +198,6 @@ static struct xen_bus_type xenbus_backend = {
 		.uevent		= xenbus_uevent_backend,
 		.probe		= xenbus_dev_probe,
 		.remove		= xenbus_dev_remove,
-		.shutdown	= xenbus_dev_shutdown,
 		.dev_groups	= xenbus_dev_groups,
 	},
 };