virtio-scsi: clean up virtio_scsi_parse_cdb

The command direction according to the guest-passed buffers
is already stored in the VirtIOSCSIReq.  We can use it instead
of computing it again from req->elem.

Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>