sound: document --streams CLI option in README

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vhost-device-sound/README.md

@@ -22,6 +22,10 @@ generated with help2man target/debug/vhost-device-sound |mandoc
      --backend <BACKEND>
             audio backend to be used [possible values: null, pipewire, alsa, gstreamer]
 
+     --streams <STREAMS>
+            comma-separated list of stream directions to enable [possible values: output, input]
+            (default: output,input)
+
      -h, --help
             Print help