qapi: convert screendump

Next commits will update devices to propagate errors.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 470f08e..6e21ddb 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -146,10 +146,7 @@
     {
         .name       = "screendump",
         .args_type  = "filename:F",
-        .params     = "filename",
-        .help       = "save screen into PPM image 'filename'",
-        .user_print = monitor_user_noop,
-        .mhandler.cmd_new = do_screen_dump,
+        .mhandler.cmd_new = qmp_marshal_input_screendump,
     },
 
 SQMP