qom: Demote already-has-a-parent to a regular error

Rather than an abort(). This allows callers to decide whether parenting
an already-parented object is a fatal error condition.

Useful for providing a default value for an object's parent in the case
where you want to set one iff it doesn't already have one.
Reviewed-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qom/object.c

@@ -1089,6 +1089,11 @@ void object_property_add_child(Object *obj, const char *name,
     gchar *type;
     ObjectProperty *op;
 
+    if (child->parent != NULL) {
+        error_setg(errp, "child object is already parented");
+        return;
+    }
+
     type = g_strdup_printf("child<%s>", object_get_typename(OBJECT(child)));
 
     op = object_property_add(obj, name, type, object_get_child_property, NULL,
@@ -1100,7 +1105,6 @@ void object_property_add_child(Object *obj, const char *name,
 
     op->resolve = object_resolve_child_property;
     object_ref(child);
-    g_assert(child->parent == NULL);
     child->parent = obj;
 
 out: