vtblk_poll_request() is used for kernel dumps and for fetching the block
device's identifier string. In the latter case, it was not calling
bus_dmamap_sync() after completing the I/O, but this is required in
general.

Thus:

• Factor out per-request code from vtblk_queue_completed().
• Use it in vtblk_poll_request() once virtqueue_poll() finishes.
• While here, assert that virtqueue_poll() returns the request that we expect.

Reported by: KMSAN
Fixes: 782105f7c898 ("vtblk: Use busdma")