CA-145685: Fix size and offset for read requests

First draft to fix the size and offset for read requests that are performed against a physical CD/DVD drive.

Considering several ways to improve this PR:
-Define a constant to replace the concrete value '512' - done
-Perform similar modifications for write requests, for consistency reasons - TBD separately
-Investigate whether other lines of code require similar modifications - TBD separately

Thanks.

Signed-off-by: Flavien Quesnel <flavien.quesnel@citrix.com>