[qemu] Default monitor/serial ports to /dev/null if -nographic.

qemu-dm runs daemonized and the default for -nographic is to redirect
the monitor and serial ports to stdio which causes qemu-dm to busy
read from stdin.  Change the default for -nographic to redirect the
monitor and serial ports to /dev/null.

Based on a patch from: Steven Smith <sos22-xen@srcf.ucam.org>
Signed-off-by: Christian Limpach <Christian.Limpach@xensource.com>

tools/ioemu/vl.c

@@ -6036,10 +6036,11 @@ int main(int argc, char **argv)
                 }
                 break;
             case QEMU_OPTION_nographic:
-                pstrcpy(monitor_device, sizeof(monitor_device), "stdio");
+                if(!strcmp(monitor_device, "vc"))
+                    pstrcpy(monitor_device, sizeof(monitor_device), "null");
                 if(!strcmp(serial_devices[0], "vc"))
                     pstrcpy(serial_devices[0], sizeof(serial_devices[0]),
-                            "stdio");
+                            "null");
                 nographic = 1;
                 break;
             case QEMU_OPTION_kernel: