xen/xenbus: make sure backend bus is registered earlier

Impact: bugfix

Need to register it before any drivers want to attach.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
[corresponds to a7a9c3a942c6 in git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xenbus/xenbus_probe_backend.c

@@ -295,5 +295,4 @@ static int __init xenbus_probe_backend_init(void)
 
 	return 0;
 }
-
-module_init(xenbus_probe_backend_init);
+subsys_initcall(xenbus_probe_backend_init);