[libvirt] Should external snapshots be possible with type volume image files?

At least up to libvirt version 1.2.16 an external snapshot fails when the image file (supported type like QCOW2) is not specified as type='file' by as type='volume' to a pool that consists of image files (type directory). The reason there is that the source element of the disk definition does not contain the full path but only references to pool and volume name. Which would require an additional indirection in order to find that it actually is a file and what the path is. And that would make things more complicated. So my question is whether this should work and needs fixing in code or was never meant to be used that way. If the latter, then maybe needs a clarification in the documentation. At least the wiki did read for me as only requiring to be a image file of a supported type. And we got at least one bug report about it. Thanks, Stefan