Reuse the tree listing of nodedev-list, coupled with the new helper
function to efficiently grab snapshot parent names, to produce
tree output for a snapshot hierarchy.  For example:

$ virsh snapshot-list dom --tree
root1
 |
  +- sibling1
  +- sibling2
  |   |
  |   +- grandchild
  |
  +- sibling3

root2
 |
  +- child

* tools/virsh.c (cmdSnapshotList): Add --tree.
* tools/virsh.pod (snapshot-list): Document it.