scsi: do not overwrite memory on REQUEST SENSE commands with a large buffer

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: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>