Upgrading CentOS 5.4 to 5.5 32Bit DomU on a Xen 64 Bit Dom0

Normally upgrading centos is very simple by doing a
yum update

But when you run centos as a 32bit DomU under a 64 Bit Dom0 server, you will receive strange error messages.
This is due to the fact, that the rpm command looks at the installed kernel, and that one is a 64bit build. So it then trys to move everything to 32 AND 64 bit.
To prevent this, just create the file /etc/rpm/platform and place the line:

i686-redhat-linux

in it.
After this, all the yum rpm commands work fine, since they are fixed to the i386/i686 platform. No confusion about the 64bit kernel.