Howto use the serial ports (/dev/ttySX) from a xen domU

I've recently done a dist-upgrade from Etch to Lenny of a xen domU (in order to use the 1.4.6 version of zabbix monitoring software), but at the reboot of the vm I noticed the "4gb seg fixup" error in console/logs.
This is the solution that worked for me:

How to connect different virtual machines to different networks in a Xen virtualization environment with bridged networking setup using 802.1q VLAN support provided by Linux.

Installation and configuration of the Xen 3.2 virtualization system with Debian GNU/Linux 4.0 (Etch)