qom: remove unused field

The "concrete_class" field of InterfaceClass is only ever written, and as far
as I can tell is not particularly useful when debugging either; remove it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qom/object.h

@@ -573,12 +573,15 @@ struct InterfaceInfo {
  *
  * The class for all interfaces.  Subclasses of this class should only add
  * virtual methods.
+ *
+ * Note that most of the fields of ObjectClass are unused (all except
+ * "type", in fact).  They are only present in InterfaceClass to allow
+ * @object_class_dynamic_cast to work with both regular classes and interfaces.
  */
 struct InterfaceClass
 {
     ObjectClass parent_class;
     /* private: */
-    ObjectClass *concrete_class;
     Type interface_type;
 };
 

qom/object.c

@@ -314,7 +314,6 @@ static void type_initialize_interface(TypeImpl *ti, TypeImpl *interface_type,
     g_free((char *)info.name);
 
     new_iface = (InterfaceClass *)iface_impl->class;
-    new_iface->concrete_class = ti->class;
     new_iface->interface_type = interface_type;
 
     ti->class->interfaces = g_slist_append(ti->class->interfaces, new_iface);