don't use kernel with initrd when installing xen instances

[Henning Sprang]

Hi,
I just had a good while of time wasted when finding this out:

When installing a xen instance with FAI, and having it configured to
use and initrd, the root file system won't get mounted. The vm hangs
on the message:

Boot: waiting for root file system

and after a while it just runs the busybox system contained in the initrd.
I finally found out that the only difference between other systems
where i made FAI installations and the one I tried to install was the
initrd, removed it, and everything was working fine.
I am not so deep into the functioning of the initrd, but probably the
root mounting process via ramdisk doesn't allow another change of the
root file system to the nfs root somehow.

In "normal" FAI install mode this problem probably doesn't occur
because either the fai-kernels don't use an initrd or they have an
initrd that don't have this problem - I didn't check that yet.

Henning