Still needs fixing upstream, but should be relatively easy to reproduce and thus locate the bug.

Right now, libvirt documents that it refuses to delete external snapshots only because it is not implemented safely; this bug represents a hole where we aren't following the documentation.  Alternatives are to patch code to match docs (quick), or to actually implement snapshot deletion (longer, but more desirable).  I see no need to do a quick patch now only to undo it later when true snapshot deletion is implemented.

Upstream proposed patch:
https://www.redhat.com/archives/libvir-list/2014-October/msg00823.html

In POST:
http://post-office.corp.redhat.com/archives/rhvirt-patches/2014-October/msg00747.html

Change to VERIFIED status. Steps as below:

# virsh snapshot-create-as rh7 internal
Domain snapshot internal created
# virsh snapshot-create-as rh7 external --disk-only
Domain snapshot external created
# virsh snapshot-delete rh7 internal --children
error: Failed to delete snapshot internal
error: unsupported configuration: deletion of 1 external disk snapshots not supported yet
# virsh snapshot-delete rh7 internal --children-only 
error: Failed to delete snapshot internal
error: unsupported configuration: deletion of 1 external disk snapshots not supported yet

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2015-0323.html