refactor(blk): offload virtqueue drain to worker thread

[agicy]

Summary

This PR refactors the virtio-blk worker threading model to eliminate the intermediate producer-consumer queue between the main thread and the I/O worker thread.

Before

Guest kicks virtqueue
       ↓
  main thread (epoll)
       ↓
  parse descriptors → push to procq (TAILQ, mutex-protected)
       ↓
  worker thread → pop from procq → perform I/O → update used ring → inject IRQ

• The main thread parsed every descriptor chain and pushed blkp_req structs onto a cross-thread queue (procq).
• The worker thread popped from the queue, performed disk I/O, and injected IRQs.
• Both threads contended on procq's mutex for every single I/O request.

After

Guest kicks virtqueue
       ↓
  main thread → signal condvar (no virtqueue access)
       ↓
  worker thread → drain virtqueue directly → perform I/O → update used ring → inject IRQ

• The main thread only signals the worker via pthread_cond_signal — it never touches the virtqueue.
• The worker thread owns the virtqueue exclusively: it parses descriptors, performs I/O, adjusts the used ring, and injects IRQs, all from a single thread.
• Synchronization is reduced to one signal/wakeup per guest notification (batch), rather than per-request lock/unlock on the procq.

Performance

Test setup: ramdisk backend, fio + io_uring, 120s runtime, RK3588.

Metric	Old	New	Change
IOPS (4k rand read, qd=64)	89.0k	93.7k	+5.3%
Throughput (1M seq read, qd=16)	3831 MiB/s	4026 MiB/s	+5.1%
P99 tail latency (4k rand, qd=64)	906 us	775 us	-14.5%
P99.99 tail latency (4k rand, qd=64)	113.8 ms	76.0 ms	-33.2%

Bug Fixes

While refactoring, several pre-existing issues were fixed:

• Crash on early close: Guard pthread_join so virtio_blk_close doesn't join an uninitialized thread handle if the worker was never started.
• Incorrect written_len on read error: When preadv fails, written_len no longer carries -1 into the used-ring update (was reporting 0 bytes despite the status byte being written).
• Missing written_len for GET_ID: Now reports correct data length via the used ring so the guest sees the full device ID string.
• close(-1) on open failure: Removed spurious close(-1) in the error path when open() fails.
• Block device size detection: When fstat returns st_size = 0 (real block devices), fall back to ioctl(BLKGETSIZE64).

[li041]

Thanks for the contribution. This update adds clearer documentation to the virtio-blk model and its related functions, making the code easier to understand and maintain. It also refactors the existing implementation by extracting the core block operations into dedicated helper functions, which improves the overall structure and readability.

Only small nit, otherwise LGTM!