xen/balloon: Move the registration from device to subsystem.

With git commit 0706802183
"xen-balloon: convert sysdev_class to a regular subsystem" we would
end up with the attributes being put in:

 /sys/devices/xen_memory0/target_kb
instead of
/sys/devices/system/xen_memory/xen_memory0/target_kb

Making the tools inable to deflate the kernel to make more space
for launching another guest and printing:
Error: Failed to query current memory allocation of dom0

Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Suggested-by:  Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xen-balloon.c

@@ -221,7 +221,7 @@ static int register_balloon(struct device *dev)
 {
 	int i, error;
 
-	error = bus_register(&balloon_subsys);
+	error = subsys_system_register(&balloon_subsys, NULL);
 	if (error)
 		return error;