scsi disk block descriptor v2

The SCSI-2 documentation suggests, that although the block
descriptor is optional for an arbitrary SCSI-2 device (chapter 8.2.10,
http://ldkelley.com/SCSI2/SCSI2/SCSI2/SCSI2/SCSI2-08.html )
it is mandatory for a disk: chapters 9.1.2, 9.3.3
( http://ldkelley.com/SCSI2/SCSI2/SCSI2/SCSI2-09.html ) don't say
"optional" any more, just "The block descriptor in the MODE SENSE
data describes the block lengths that are used on the medium."

v2: limit the number of sectors reported in the block descriptor to 24 bits.

Signed-off-by: Artyom Tarasenko <atar4qemu@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
