I'm aware that the autobuild minimal installers carry support for virtio disk drives so that they can be used from within a kvm guest. However I'm much more likely to carry and use the latest and previous livedvd versions with me when doing installs and diagnostics. This tendency also applies for me to use the livedvd iso when working with a guest vm in virt-manager. Besides, the livedvd has a gui while the minimals do not.
There must be something in your workflow for a livedvd build (genkernel maybe?) which causes you to leave out virtio support in the resulting kernel. I had written this up on the 2011 dvd, and it was subsequently fixed in 2011.1, but now its missing again for 2012.
12.1 should have this added.
please check 12.1 for this.