libxl: Replace short-form boolean for QEMU's -vnc

f3f778c817 forgot one boolean parameter.

Fixes: f3f778c817 ("libxl: Replace QEMU's command line short-form boolean option")
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>

tools/libs/light/libxl_dm.c

@@ -1324,7 +1324,7 @@ static int libxl__build_device_model_args_new(libxl__gc *gc,
             vncarg = GCSPRINTF("127.0.0.1:%d", vnc->display);
 
         if (vnc->passwd && vnc->passwd[0]) {
-            vncarg = GCSPRINTF("%s,password", vncarg);
+            vncarg = GCSPRINTF("%s,password=on", vncarg);
         }
 
         if (libxl_defbool_val(vnc->findunused)) {