default devices: add global cmd line option.

Add global command line option to disable default devices.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/vl.c b/vl.c
index 444f414..69b577f 100644
--- a/vl.c
+++ b/vl.c
@@ -5471,6 +5471,12 @@
             case QEMU_OPTION_incoming:
                 incoming = optarg;
                 break;
+            case QEMU_OPTION_nodefaults:
+                default_serial = 0;
+                default_parallel = 0;
+                default_monitor = 0;
+                default_vga = 0;
+                break;
 #ifndef _WIN32
             case QEMU_OPTION_chroot:
                 chroot_dir = optarg;