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

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

This fixes thin provisioning on glusterfs.

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