It was reported that libvirt did not perform a proper clean up after volume creation fails on an NFS pool with "root_squash" option enabled. This leaves the NFS pool in damaged state.
Fix is public as of 2015-08-02: https://www.redhat.com/archives/libvir-list/2015-September/msg00018.html
Libvirt Security Notice: http://security.libvirt.org/2015/0003.html
Upstream patches were released, see the Security Notice above.