Hello Timo,for some reason I didn't received your question and I found it just now on bugs.debian.org pages.
I didn't record my previous commands but I used standard xm block-attach syntax from vhost config like:
'xm block-attach <domain> phy:data/disk /dev/xvda6 w' I am mapping LVM LV's from VG named 'data' to my vhosts. -- Peter Viskup