There are two parts to the serial-console-game:
1) vm base config (guest hardware config)
2) vm software config

For the guest hardware config, you can either hand-edit all vms as they are built (sucky) or simply edit the template used. Templates are stored in /etc/vmbuilder/libvirt, I added a devices/serial section like:

<serial type="pty">
<source path="/dev/pts/3" />
<target port="1" />

inside the section. This creates a serial interface on the guest hardware.

For the vm software config, simply have the first-boot script install /etc/init/tty0.conf for you!