Previously, to get the name of all snapshots with children, it was
necessary to get the name of all snapshots and then remove the
name of leaf snapshots.  This is racy, and somewhat inefficient
compared to planned API additions.  We can emulate --no-metadata on
0.9.5-0.9.12, but for now, there is no emulation of --no-leaves.

* tools/virsh.c (cmdSnapshotList): Add new options --no-leaves and
--no-metadata.
(vshSnapshotList): Emulate where possible.
* tools/virsh.pod (snapshot-list): Document them.