I started this on the wrong list[1], but basically, guests typically don't have the
kernel bridge module loaded and ncftool fails to run in this situation. I opened a
bug[2].
[1]
http://www.redhat.com/archives/virt-tools-list/2009-November/msg00029.html
[2]
https://bugzilla.redhat.com/show_bug.cgi?id=536895