snapshot: new virsh function factored from snapshot-list

snapshot: new virsh function factored from snapshot-list

This patch is based on the fallback code out of cmdSnapshotList,
with tweaks to keep the snapshot objects around rather than just
their name, and to remove unwanted elements before returning.
It looks forward to a future patch when we add a way to list all
snapshot objects at once, and the next patch will simplify
cmdSnapshotList to take advantage of this factorization.

* tools/virsh.c (vshSnapshotList, vshSnapshotListFree): New functions.