monitor: improve auto complete of "help" for single command in sub group

Now special case "help *" in auto completion can work with sub commands,
such as "help info u*".

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

monitor.c

diff --git a/monitor.c b/monitor.c
index 721c74d..0aeaf6c 100644
--- a/monitor.c
+++ b/monitor.c
@@ -4330,10 +4330,8 @@
                     cmd_completion(mon, str, QKeyCode_lookup[i]);
                 }
             } else if (!strcmp(cmd->name, "help|?")) {
-                readline_set_completion_index(mon->rs, strlen(str));
-                for (cmd = cmd_table; cmd->name != NULL; cmd++) {
-                    cmd_completion(mon, str, cmd->name);
-                }
+                monitor_find_completion_by_table(mon, cmd_table,
+                                                 &args[1], nb_args - 1);
             }
             break;
         default: