... so I do not forget the next time I have to figure this stuff out. And perhaps to help other poor souls fiddling with v6/v4 xen setups in a Hetzner network environment.
You can basically follow along the Xen Project Beginners Guide.
Install Debian Wheezy via Hetzner's installimage
on the rescue system, the only important part about partitioning is that you have an LVM volume group named vg0
with enough space for your guests' disks.