scsi-disk: fix reads from scsi-disk devices

Commit fcaafb10 accidentally broke reads from
scsi-disk devices when being updated from its original form to use the new
byte-based block functions. Add the extra missing sector to offset conversion
in order to restore read functionality.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: xiaoqiang zhao <zxq_yx_007@163.com>
Message-id: 1464931021-25117-1-git-send-email-mark.cave-ayland@ilande.co.uk
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>