Disable serial logging by default

The serial console logging causes VirtualBox to consume an immense amount of CPU, making coreos-vagrant nearly unusable. Disabling it by default treats the problem until VirtualBox can fix the problem
upstream.

Thanks to @DieterReuter who pointed this out: https://github.com/coreos/coreos-vagrant/commit/1a212b1dc8192a76279d1f08f17f97a29bede209#commitcomment-6057110