X server: Log to the journal instead of /var/log/X.0.log

This ensures that "journalctl -u display-manager" does what you would
expect in 2016. However, the main reason is to ensure that our VM
tests show the output of the X server.

A slight problem is that with KDE user switching, messages from the
various X servers end up in the same place. However, that's an
improvement over the previous situation, where the second X server
would overwrite the /var/log/X.0.log of the first. (This was caused by
the fact that we were passing a hard-coded value for -logfile.)