qom: Fix comment typo

It's qom_unref, not qdef_unref.

Signed-off-by: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>

include/qom/object.h

@@ -901,7 +901,7 @@ GSList *object_class_get_list(const char *implements_type,
 void object_ref(Object *obj);
 
 /**
- * qdef_unref:
+ * object_unref:
  * @obj: the object
  *
  * Decrease the reference count of a object.  A object cannot be freed as long