scsi-disk: fix the block descriptor returned by the MODE SENSE command

The block descriptor contains the number of blocks, not the highest LBA.
Real hard disks return 0 if the number of blocks exceed the maximum 0xFFFFFF.

SCSI-Spec:
http://ldkelley.com/SCSI2/SCSI2/SCSI2-08.html#8.3.3
"The number of blocks field specifies the number of logical blocks on the
medium to which the density code and block length fields apply. A value
of zero indicates that all of the remaining logical blocks of the logical
unit shall have the medium characteristics specified."
Signed-off-by: NBernhard Kohl <bernhard.kohl@nsn.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

hw/scsi-disk.c

@@ -662,9 +662,8 @@ static int scsi_disk_emulate_mode_sense(SCSIRequest *req, uint8_t *outbuf)
             outbuf[7] = 8; /* Block descriptor length  */
         }
         nb_sectors /= s->cluster_size;
-        nb_sectors--;
         if (nb_sectors > 0xffffff)
-            nb_sectors = 0xffffff;
+            nb_sectors = 0;
         p[0] = 0; /* media density code */
         p[1] = (nb_sectors >> 16) & 0xff;
         p[2] = (nb_sectors >> 8) & 0xff;