sysbus: Fix latent bug with onboard devices

The first call of sysbus_get_default() creates the main system bus and
stores it in QOM as "/machine/unattached/sysbus".  This must not
happen before main() creates "/machine", or else container_get() would
"helpfully" create it as "container" object, and the real creation of
"/machine" would later abort with "attempt to add duplicate property
'machine' to object (type 'container')".  Has been that way ever since
we wired up busses in QOM (commit f968fc6892, v1.2.0).

I believe the bug is latent.  I got it to bite by trying to
qdev_create() a sysbus device from a machine's .instance_init()
method.

The fix is obvious: store the main system bus in QOM right after
creating "/machine".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190308131445.17502-5-armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

hw/core/sysbus.c

@@ -357,9 +357,6 @@ static void main_system_bus_create(void)
     qbus_create_inplace(main_system_bus, system_bus_info.instance_size,
                         TYPE_SYSTEM_BUS, NULL, "main-system-bus");
     OBJECT(main_system_bus)->free = g_free;
-    object_property_add_child(container_get(qdev_get_machine(),
-                                            "/unattached"),
-                              "sysbus", OBJECT(main_system_bus), NULL);
 }
 
 BusState *sysbus_get_default(void)

vl.c

@@ -3989,6 +3989,10 @@ int main(int argc, char **argv, char **envp)
     }
     object_property_add_child(object_get_root(), "machine",
                               OBJECT(current_machine), &error_abort);
+    object_property_add_child(container_get(OBJECT(current_machine),
+                                            "/unattached"),
+                              "sysbus", OBJECT(sysbus_get_default()),
+                              NULL);
 
     if (machine_class->minimum_page_bits) {
         if (!set_preferred_target_page_bits(machine_class->minimum_page_bits)) {