scsi-disk: add UNMAP limits to block limits VPD page

Linux prefers WRITE SAME to UNMAP if the limits are zero, and WRITE
SAME does not discard anything unless the device can guarantee that
the resulting block is zero.

Setting the maximum unmap block and descriptor counts to non-zero
makes Linux choose UNMAP and fixes thin provisioning on glusterfs.

While the maximum unmap block count can have some effect on performance,
the (suggested) maximum number of descriptors is not particularly
important so I didn't add a customization option.  SCSI drivers are
used to online firmware updates so I'm not yet adding versioning support
for SCSI, but we're probably getting close to the point when it's worth
thinking about it.

Reported-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>