qapi: Convert migrate_set_downtime

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 5eb4b76..b4cadeb 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -518,10 +518,7 @@
     {
         .name       = "migrate_set_downtime",
         .args_type  = "value:T",
-        .params     = "value",
-        .help       = "set maximum tolerated downtime (in seconds) for migrations",
-        .user_print = monitor_user_noop,
-        .mhandler.cmd_new = do_migrate_set_downtime,
+        .mhandler.cmd_new = qmp_marshal_input_migrate_set_downtime,
     },
 
 SQMP