I have two servers with KVM running, on both I use preseeding to install the guests. It's amazing to watch a guest install automatically by using virt-install and passing some parameters on command line. Basically you need to pass further preseeding parameters in a file. When converting this script to wheezy I ran into a problem. In squeeze, the parameter that defines the disk was named vda (why vda??? however) in wheezy its sda now (more like its in every debian install). Means I had to change the config parameter from

d-i partman-auto/disk string /dev/vda
d-i partman-auto/disk string /dev/sda