Libvirt no longer builds on FreeBSD 8.3. I had brought this up awhile ago, but didn't have the resources to get the requested information. A fellow committer was able to build, and send me an archive of the complete build failure for analysis.
Here is a link to build archive for investigation.http://people.freebsd.org/~jgh/files/libvirt_83amd64.tar.xz
Please let me know if there is any information you need.