Fix #1999: cli: listsnapshot: handle multiple roots and mark orphaned as root

Fix #1999: cli: listsnapshot: handle multiple roots and mark orphaned as root

This commit addresses the following things:

* There can be multiple independent snapshot tree roots, display them
  all
* If a snapshot defines a parent that doesn't exist, it is now
  considered a snapshot root

There is a potential issue (which was also before and also affects the
GUI): circular snapshot trees.  That plays into the second mentioned
issue above.  If you manage to have a snapshot that defines a
non-existing root in the config, and then create a snapshot with that
exact name as a child of that snapshot, it would create a circular
dependency.  This would have to get addressed in the GUI too.

Signed-off-by: Rhonda D'Vine <rhonda@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>