Other scsi_target_reqops commands were careful about not using r->cmd.xfer
directly, and instead always cap it to a fixed length.  This was not done
for REQUEST SENSE, and this patch fixes it.
Reported-by: NBlue Swirl <blauwirbel@gmail.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>