uapi/virtio_scsi: allow overriding CDB/SENSE size

QEMU wants to use virtio scsi structures with
a different VIRTIO_SCSI_CDB_SIZE/VIRTIO_SCSI_SENSE_SIZE,
let's add ifdefs to allow overriding them.

Keep the old defines under new names:
VIRTIO_SCSI_CDB_DEFAULT_SIZE/VIRTIO_SCSI_SENSE_DEFAULT_SIZE,
since that's what these values really are:
defaults for cdb/sense size fields.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/uapi/linux/virtio_scsi.h

@@ -29,8 +29,16 @@
 
 #include <linux/virtio_types.h>
 
-#define VIRTIO_SCSI_CDB_SIZE   32
-#define VIRTIO_SCSI_SENSE_SIZE 96
+/* Default values of the CDB and sense data size configuration fields */
+#define VIRTIO_SCSI_CDB_DEFAULT_SIZE   32
+#define VIRTIO_SCSI_SENSE_DEFAULT_SIZE 96
+
+#ifndef VIRTIO_SCSI_CDB_SIZE
+#define VIRTIO_SCSI_CDB_SIZE VIRTIO_SCSI_CDB_DEFAULT_SIZE
+#endif
+#ifndef VIRTIO_SCSI_SENSE_SIZE
+#define VIRTIO_SCSI_SENSE_SIZE VIRTIO_SCSI_SENSE_DEFAULT_SIZE
+#endif
 
 /* SCSI command request, followed by data-out */
 struct virtio_scsi_cmd_req {