macio: Fix macio-bus to be a subtype of System bus

The devices we plug into the macio-bus are all sysbus devices
(DeviceClass member bus_type is TYPE_SYSTEM_BUS), but macio-bus does
not derive from TYPE_SYSTEM_BUS.  Fix that.

"info qtree" now shows the devices' mmio ranges, as it should

Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: qemu-ppc@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200609122339.937862-16-armbru@redhat.com>

hw/misc/macio/macio.c

@@ -492,7 +492,7 @@ static void macio_class_init(ObjectClass *klass, void *data)
 
 static const TypeInfo macio_bus_info = {
     .name = TYPE_MACIO_BUS,
-    .parent = TYPE_BUS,
+    .parent = TYPE_SYSTEM_BUS,
     .instance_size = sizeof(MacIOBusState),
 };