scsi-disk: Don't enlarge min_io_size to max_io_size

Some backends report big max_io_sectors. Making min_io_size the same
value in this case will make it impossible for guest to align memory,
therefore the disk may not be usable at all.

Do not enlarge them when they are zero.
Reported-by: NDavid Gibson <dgibson@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>

Message-Id: <20180327164141.19075-1-famz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi/scsi-disk.c

@@ -714,10 +714,12 @@ static int scsi_disk_emulate_inquiry(SCSIRequest *req, uint8_t *outbuf)
 
                 /* min_io_size and opt_io_size can't be greater than
                  * max_io_sectors */
-                min_io_size =
-                    MIN_NON_ZERO(min_io_size, max_io_sectors);
-                opt_io_size =
-                    MIN_NON_ZERO(opt_io_size, max_io_sectors);
+                if (min_io_size) {
+                    min_io_size = MIN(min_io_size, max_io_sectors);
+                }
+                if (opt_io_size) {
+                    opt_io_size = MIN(opt_io_size, max_io_sectors);
+                }
             }
             /* required VPD size with unmap support */
             buflen = 0x40;