default devices: add global cmd line option.

Add global command line option to disable default devices.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qemu-options.hx

@@ -1881,6 +1881,11 @@ DEF("incoming", HAS_ARG, QEMU_OPTION_incoming, \
 STEXI
 ETEXI
 
+DEF("nodefaults", 0, QEMU_OPTION_nodefaults, \
+    "-nodefaults     don't create default devices.\n")
+STEXI
+ETEXI
+
 #ifndef _WIN32
 DEF("chroot", HAS_ARG, QEMU_OPTION_chroot, \
     "-chroot dir     Chroot to dir just before starting the VM.\n")

vl.c

@@ -5471,6 +5471,12 @@ int main(int argc, char **argv, char **envp)
             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;