monitor: allow to disable the default monitor

Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

qemu-options.hx

@@ -2528,6 +2528,7 @@ Redirect the monitor to host device @var{dev} (same devices as the
 serial port).
 The default device is @code{vc} in graphical mode and @code{stdio} in
 non graphical mode.
+Use @code{-monitor none} to disable the default monitor.
 ETEXI
 DEF("qmp", HAS_ARG, QEMU_OPTION_qmp, \
     "-qmp dev        like -monitor but opens in 'control' mode\n",

vl.c

@@ -3366,8 +3366,10 @@ int main(int argc, char **argv, char **envp)
                     break;
                 }
             case QEMU_OPTION_monitor:
-                monitor_parse(optarg, "readline");
                 default_monitor = 0;
+                if (strncmp(optarg, "none", 4)) {
+                    monitor_parse(optarg, "readline");
+                }
                 break;
             case QEMU_OPTION_qmp:
                 monitor_parse(optarg, "control");