scsi-bus: prepare scsi_req_new for introduction of parse_cdb

The per-SCSIDevice parse_cdb callback must not be called if the
request will go through special SCSIReqOps, so detect the special
cases early enough.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>