snapshot: take advantage of new relations

snapshot: take advantage of new relations

Among other improvements, virDomainSnapshotForEachDescendant is
changed from iterative O(n^2) to recursive O(n).  A bit better
than the O(n^3) implementation in virsh snapshot-list!

* src/conf/domain_conf.c (virDomainSnapshotObjListNum)
(virDomainSnapshotObjListNumFrom)
(virDomainSnapshotObjeListGetNames, virDomainSnapshotForEachChild)
(virDomainSnapshotForEachDescendant): Optimize.
(virDomainSnapshotActOnDescendant): Tweak.
(virDomainSnapshotActOnChild, virDomainSnapshotMarkDescendant):
Delete, now that they are unused.