Monitor: Convert do_screen_dump() to QObject

Trivial, as it never fails, doesn't have output nor return
any data.

Note that it's also available under QMP, as kvm-autotest
needs this.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

monitor.c

@@ -1164,9 +1164,10 @@ static int do_change(Monitor *mon, const QDict *qdict, QObject **ret_data)
     return ret;
 }
 
-static void do_screen_dump(Monitor *mon, const QDict *qdict)
+static int do_screen_dump(Monitor *mon, const QDict *qdict, QObject **ret_data)
 {
     vga_hw_screen_dump(qdict_get_str(qdict, "filename"));
+    return 0;
 }
 
 static void do_logfile(Monitor *mon, const QDict *qdict)

qemu-monitor.hx

@@ -199,7 +199,8 @@ ETEXI
         .args_type  = "filename:F",
         .params     = "filename",
         .help       = "save screen into PPM image 'filename'",
-        .mhandler.cmd = do_screen_dump,
+        .user_print = monitor_user_noop,
+        .mhandler.cmd_new = do_screen_dump,
     },
 
 STEXI