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: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>