Don't set default monitor when there is a mux'ed one

This fixes eg. "-nographic -serial mon:stdio [-serial ...]".
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

vl.c

@@ -5377,6 +5377,9 @@ int main(int argc, char **argv, char **envp)
             case QEMU_OPTION_serial:
                 add_device_config(DEV_SERIAL, optarg);
                 default_serial = 0;
+                if (strncmp(optarg, "mon:", 4) == 0) {
+                    default_monitor = 0;
+                }
                 break;
             case QEMU_OPTION_watchdog:
                 if (watchdog) {
@@ -5395,10 +5398,16 @@ int main(int argc, char **argv, char **envp)
             case QEMU_OPTION_virtiocon:
                 add_device_config(DEV_VIRTCON, optarg);
                 default_virtcon = 0;
+                if (strncmp(optarg, "mon:", 4) == 0) {
+                    default_monitor = 0;
+                }
                 break;
             case QEMU_OPTION_parallel:
                 add_device_config(DEV_PARALLEL, optarg);
                 default_parallel = 0;
+                if (strncmp(optarg, "mon:", 4) == 0) {
+                    default_monitor = 0;
+                }
                 break;
             case QEMU_OPTION_debugcon:
                 add_device_config(DEV_DEBUGCON, optarg);