selftests: ublk: add ublk zero copy test

Enable zero copy on file backed target, meantime add one fio test for
covering write verify, another test for mkfs/mount/umount.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250228161919.2869102-4-ming.lei@redhat.com


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