This function looks like a small optimization. I don't think that this optimization makes sense because it makes the code harder to read for no value.
So IMHO, we should drop it. E.g. just use:

static inline uint16_t
virtio_htog16(bool modern, uint16_t val)
{
  if (modern)
    return (htole16(val));
  else
    return (val);
}

I agree with Corvin's suggestion and think it is more readable.