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:

1. Verify that in your domU you have libc6-xen installed:

   monitor:~# dpkg -l libc6-xen
   [...]
   ii  libc6-xen    2.7-18    GNU C Library: Shared libraries [Xen version]
   

2. On the domU if you have /lib/tls directory rename it:

   mv /lib/tls /lib/tls.disabled
   

3. On the domU Edit the /etc/ld.so.conf.d/libc6-xen.conf end specify:

   hwcap 0 nosegneg
   

4. On the domU run ldconfig:

   monitor:~# ldconfig
   

5. Reboot the domU

   monitor:~# reboot