selftests: ublk: support target specific command line

Support target specific command line for making related command line code
handling more readable & clean.

Also helps for adding new features.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-11-ming.lei@redhat.com


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