monitor: allow to disable the default monitor

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/vl.c b/vl.c
index 59dc0b4..510d2c2 100644
--- a/vl.c
+++ b/vl.c
@@ -3366,8 +3366,10 @@
                     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");