Bug introduced in commit 35abced2.  On an inactive domain,
$ virsh snapshot-create-as dom snap
$ virsh snapshot-create dom
$ virsh snapshot-create dom
$ virsh snapshot-delete --children dom snap
could crash libvirtd, due to a use-after-free that results
when the callback freed the current element in the iteration.

* src/conf/domain_conf.c (virDomainSnapshotForEachChild)
(virDomainSnapshotActOnDescendant): Allow iteration to delete
current child.