xen: remove DEFINE_XENBUS_DRIVER() macro

The DEFINE_XENBUS_DRIVER() macro looks a bit weird and causes sparse
errors.

Replace the uses with standard structure definitions instead.  This is
similar to pci and usb device registration.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
diff --git a/drivers/xen/xen-scsiback.c b/drivers/xen/xen-scsiback.c
index ad11258..3e32146 100644
--- a/drivers/xen/xen-scsiback.c
+++ b/drivers/xen/xen-scsiback.c
@@ -2056,11 +2056,12 @@
 	{ "" }
 };
 
-static DEFINE_XENBUS_DRIVER(scsiback, ,
+static struct xenbus_driver scsiback_driver = {
+	.ids			= scsiback_ids,
 	.probe			= scsiback_probe,
 	.remove			= scsiback_remove,
 	.otherend_changed	= scsiback_frontend_changed
-);
+};
 
 static void scsiback_init_pend(void *p)
 {