Libvirt does not yet support snapshot revert for external snapshots; there have been some ideas posted on list about what needs to be done to add the support, but so far no finalized patches. It's a bug that your attempts to revert are not being more vocal about failing due to lack of support. For now, you'll have to do the revert by manual use of qemu-img and editing of a libvirt <domain> XML; more details can be found on this wiki page (still in a bit of disarray, so feel free to edit it for clarity according to what works for you):
I am not able to find a way to revert an external snapshot created by "virsh snapshot-create-as $dom $name --live --memspec /path/to/memoryfile" using qemu-img. Could you give us some specific instructions on how to revert an external snapshot manually? Xinyang -- Xinyang GE Department of Computer Science & Engineering The Pennsylvania State University Homepage: http://www.cse.psu.edu/~xxg113/