Doesn't this imply that other image formats are supported?:
http://pkgs.fedoraproject.org/gitweb/?p=python-virtinst.git;a=commit;h=1045f...
I actually got this done over the weekend, I just have pushed it up to the master branch. Part of the issue I found, was that when I set the disk to qcow the VM wouldn't boot, and when I set it to qcow2 virtinst ignored it and defaulted it back to raw (I assume because it's not one of the listed TAP drivers).
So, I can push this up, but YMMV in actually trying to use it. For the record, the #kvm guys did say that raw was the best/fastest option, but this functionality could be useful in the future.