qdev: Don't assume existence of parent bus on unparenting

Commit 667d22d1 (qdev: move bus removal
to object_unparent) made the assumption that at unparenting time
parent_bus is not NULL. This assumption is unjustified since
object_unparent() may well be called directly after object_initialize(),
without any qdev_set_parent_bus().

This did not cause any issues yet because qdev_[try_]create() does call
qdev_set_parent_bus(), falling back to SysBus if unsupplied.

While at it, ensure that this new function uses the device_ prefix and
make the name more neutral in light of this semantic change.
Reported-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Tested-by: NIgor Mammedov <imammedo@redhat.com>

hw/qdev.c

@@ -698,16 +698,18 @@ static void device_class_base_init(ObjectClass *class, void *data)
     klass->props = NULL;
 }
 
-static void qdev_remove_from_bus(Object *obj)
+static void device_unparent(Object *obj)
 {
     DeviceState *dev = DEVICE(obj);
 
-    bus_remove_child(dev->parent_bus, dev);
+    if (dev->parent_bus != NULL) {
+        bus_remove_child(dev->parent_bus, dev);
+    }
 }
 
 static void device_class_init(ObjectClass *class, void *data)
 {
-    class->unparent = qdev_remove_from_bus;
+    class->unparent = device_unparent;
 }
 
 void device_reset(DeviceState *dev)