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>