vhost: logging thinko fix

vhost was dong some complex math to get
offset to log at, and got it wrong by a couple of bytes,
while in fact it's simple: get address where we write,
subtract start of buffer, add log base.

Do it this way.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>