selftests: ublk: simplify loop io completion

Use the added target io handling helpers for simplifying loop io
completion.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250322093218.431419-8-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>