crypto: expose encryption sector size in APIs

While current encryption schemes all have a fixed sector size of
512 bytes, this is not guaranteed to be the case in future. Expose
the sector size in the APIs so the block layer can remove assumptions
about fixed 512 byte sectors.

Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170927125340.12360-3-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
