crypto/block: refactor qcrypto_block_*crypt_helper functions

qcrypto_block_encrypt_helper and qcrypto_block_decrypt_helper are
almost identical, let's reduce code duplication and simplify further
improvements.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>