monitor: allow register hmp commands

Allow commands having a NULL cmd pointer, add a function to set the
pointer later.  Use case: allow modules implement hmp commands.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jose R. Ziviani <jziviani@suse.de>
Message-Id: <20210624103836.2382472-31-kraxel@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/monitor/hmp.c b/monitor/hmp.c
index 6c0b33a..d50c312 100644
--- a/monitor/hmp.c
+++ b/monitor/hmp.c
@@ -1089,6 +1089,13 @@
         return;
     }
 
+    if (!cmd->cmd) {
+        /* FIXME: is it useful to try autoload modules here ??? */
+        monitor_printf(&mon->common, "Command \"%.*s\" is not available.\n",
+                       (int)(cmdline - cmd_start), cmd_start);
+        return;
+    }
+
     qdict = monitor_parse_arguments(&mon->common, &cmdline, cmd);
     if (!qdict) {
         while (cmdline > cmd_start && qemu_isspace(cmdline[-1])) {