block/export: rewrite vduse-blk drain code

vduse_blk_detach_ctx() waits for in-flight requests using
AIO_WAIT_WHILE(). This is not allowed according to a comment in
bdrv_set_aio_context_commit():

  /*
   * Take the old AioContex when detaching it from bs.
   * At this point, new_context lock is already acquired, and we are now
   * also taking old_context. This is safe as long as bdrv_detach_aio_context
   * does not call AIO_POLL_WHILE().
   */

Use this opportunity to rewrite the drain code in vduse-blk:

- Use the BlockExport refcount so that vduse_blk_exp_delete() is only
  called when there are no more requests in flight.

- Implement .drained_poll() so in-flight request coroutines are stopped
  by the time .bdrv_detach_aio_context() is called.

- Remove AIO_WAIT_WHILE() from vduse_blk_detach_ctx() to solve the
  .bdrv_detach_aio_context() constraint violation. It's no longer
  needed due to the previous changes.

- Always handle the VDUSE file descriptor, even in drained sections. The
  VDUSE file descriptor doesn't submit I/O, so it's safe to handle it in
  drained sections. This ensures that the VDUSE kernel code gets a fast
  response.

- Suspend virtqueue fd handlers in .drained_begin() and resume them in
  .drained_end(). This eliminates the need for the
  aio_set_fd_handler(is_external=true) flag, which is being removed from
  QEMU.

This is a long list but splitting it into individual commits would
probably lead to git bisect failures - the changes are all related.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230516190238.8401-14-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

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