monitor: move the cur_mon hack deeper for QMP

In monitor_qmp_read(), we have the hack to temporarily replace the
cur_mon pointer.  Now we move this hack deeper inside the QMP dispatcher
routine since the Monitor pointer can be actually obtained using
container_of() upon the parser object, just like most of the other JSON
parser users do.

This does not make much sense as a single patch.  However, this will be
a big step for the next patch, when the QMP dispatcher routine will be
split from the QMP parser.
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-7-peterx@redhat.com>
[eblake: rebase context of qobject_to() macro]
Signed-off-by: NEric Blake <eblake@redhat.com>

monitor.c

@@ -3765,7 +3765,9 @@ static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens)
 {
     QObject *req, *rsp = NULL, *id = NULL;
     QDict *qdict = NULL;
-    Monitor *mon = cur_mon;
+    MonitorQMP *mon_qmp = container_of(parser, MonitorQMP, parser);
+    Monitor *old_mon, *mon = container_of(mon_qmp, Monitor, qmp);
+
     Error *err = NULL;
 
     req = json_parser_parse_err(tokens, NULL, &err);
@@ -3790,8 +3792,13 @@ static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens)
         QDECREF(req_json);
     }
 
+    old_mon = cur_mon;
+    cur_mon = mon;
+
     rsp = qmp_dispatch(cur_mon->qmp.commands, req);
 
+    cur_mon = old_mon;
+
     if (mon->qmp.commands == &qmp_cap_negotiation_commands) {
         qdict = qdict_get_qdict(qobject_to(QDict, rsp), "error");
         if (qdict
@@ -3828,13 +3835,9 @@ err_out:
 
 static void monitor_qmp_read(void *opaque, const uint8_t *buf, int size)
 {
-    Monitor *old_mon = cur_mon;
-
-    cur_mon = opaque;
-
-    json_message_parser_feed(&cur_mon->qmp.parser, (const char *) buf, size);
+    Monitor *mon = opaque;
 
-    cur_mon = old_mon;
+    json_message_parser_feed(&mon->qmp.parser, (const char *) buf, size);
 }
 
 static void monitor_read(void *opaque, const uint8_t *buf, int size)