monitor: introduce monitor_qmp_respond()

A tiny refactoring, preparing to split the QMP dispatcher away.
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-12-peterx@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
[eblake: rebase to qobject_to() usage]
Signed-off-by: NEric Blake <eblake@redhat.com>

monitor.c

@@ -3822,6 +3822,38 @@ static int monitor_can_read(void *opaque)
     return (mon->suspend_cnt == 0) ? 1 : 0;
 }
 
+/*
+ * 1. This function takes ownership of rsp, err, and id.
+ * 2. rsp, err, and id may be NULL.
+ * 3. If err != NULL then rsp must be NULL.
+ */
+static void monitor_qmp_respond(Monitor *mon, QObject *rsp,
+                                Error *err, QObject *id)
+{
+    QDict *qdict = NULL;
+
+    if (err) {
+        assert(!rsp);
+        qdict = qdict_new();
+        qdict_put_obj(qdict, "error", qmp_build_error_object(err));
+        error_free(err);
+        rsp = QOBJECT(qdict);
+    }
+
+    if (rsp) {
+        if (id) {
+            /* This is for the qdict below. */
+            qobject_incref(id);
+            qdict_put_obj(qobject_to(QDict, rsp), "id", id);
+        }
+
+        monitor_json_emitter(mon, rsp);
+    }
+
+    qobject_decref(id);
+    qobject_decref(rsp);
+}
+
 static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens)
 {
     QObject *req, *rsp = NULL, *id = NULL;
@@ -3873,24 +3905,8 @@ static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens)
     }
 
 err_out:
-    if (err) {
-        qdict = qdict_new();
-        qdict_put_obj(qdict, "error", qmp_build_error_object(err));
-        error_free(err);
-        rsp = QOBJECT(qdict);
-    }
+    monitor_qmp_respond(mon, rsp, err, id);
 
-    if (rsp) {
-        if (id) {
-            qdict_put_obj(qobject_to(QDict, rsp), "id", id);
-            id = NULL;
-        }
-
-        monitor_json_emitter(mon, rsp);
-    }
-
-    qobject_decref(id);
-    qobject_decref(rsp);
     qobject_decref(req);
 }