Comment 3 for bug 221552

Here's a patch which should correct this problem. The dicts which contained the settings for everything except Linux were missing the virtio_blk and virtio_net keys. It would also be possible to change the behavior of the get_virtio_{net|blk} functions to return the default if the ostype didn't have the key, but assuming the os_type has all the options seems to be more inline with the rest of the program.

After this I was able to create and start windows-winxp and unix-freebsd6 variants.