Implement VirtIO sound device capture

[Cuda-Chen]

Summary by cubic

Implements VirtIO sound capture (RX) with PortAudio input, RX virtqueue handling, and circular buffering so input audio can be delivered to the guest. Also exposes separate input/output streams with accurate device info.

• New Features
  • Added RX virtqueue handlers (normal/flush) with default flush stream_id = 1.
  • Opened PortAudio input stream and RX callback to capture frames and enqueue to the driver.
  • Implemented circular intermediate buffer (buf_sz, buf_idx) and RX enqueue/dequeue helpers.
  • Exposed two streams (output + input), mono S16, with per-stream jack/pcm/chmap info.
  • Added RX worker thread, mutex/cond vars, and VSND_QUEUE_RX notifications.
  • Applied direction-aware flush in pcm_release and set RX response length/status.

^{Written for commit 4dd617c. Summary will update automatically on new commits.}

[jserv]

Rebase to avoid unnecessary merge commits.