qcrypto-luks: rename some fields in QCryptoBlockLUKSHeader

* key_bytes -> master_key_len
* payload_offset = payload_offset_sector (to emphasise that this isn't byte offset)
* key_offset -> key_offset_sector - same as above for luks slots

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>