scsi: Advertise limits by blocksize, not 512

s->blocksize may be larger than 512, in which case our
tweaks to max_xfer_len and opt_xfer_len must be scaled
appropriately.

CC: qemu-stable@nongnu.org
Reported-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>