[Virtio-fs] [virtiofsd] MR closed: Draft: process_queue_pool: Prevent queue starvation

In `process_queue_pool()`, instead of processing the queue until
it's empty, we collect the entries from the queue once and then
process only those entries.  This prevents queues from starving
other queues. Without this commit, `process_queue_pool()` might
never return, preventing all other queues from making progress:
e.g., the REQ queue might prevent the HIPRIO queue from making
progress.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/90