Operating on a list of snapshot objects looks so much simpler.
In particular, since the helper function already trimmed out
irrelevant entries, we no longer have quite so many special cases
on finding the first snapshot to operate on.  Also, vshTreePrint
no longer has a generic callback struct; both clients now pass
something different according to their own needs.

* tools/virsh.c (cmdSnapshotList): Use previous patches.
(vshTreeArrayLookup): Rename...
(vshNodeListLookup): ...now that it only has one client.
(cmdNodeListDevices): Adjust caller.