selftests: ublk: support user recovery

Add user recovery feature.

Meantime add user recovery test: generic_04 and generic_05(zero copy)

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-12-ming.lei@redhat.com


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