monitor: Use argument type 'O' for device_add

While there, improve the params help text.
diff --git a/qemu-monitor.hx b/qemu-monitor.hx
index 7f9d261..45e5fd1 100644
--- a/qemu-monitor.hx
+++ b/qemu-monitor.hx
@@ -570,8 +570,8 @@
 
     {
         .name       = "device_add",
-        .args_type  = "config:s",
-        .params     = "device",
+        .args_type  = "device:O",
+        .params     = "driver[,prop=value][,...]",
         .help       = "add device, like -device on the command line",
         .mhandler.cmd = do_device_add,
     },