snapshot: implement snapshot roots listing in vbox

snapshot: implement snapshot roots listing in vbox

Commit 9f5e53e introduced the ability to filter snapshots to
just roots, but it was never implemented for VBox until now.

The VBox implementation prohibits deletion of a snapshot with
multiple children.  Hence, there can only be at most one root,
which is found by searching for the snapshot with a NULL uuid.

Prior to 4.0, snapshotGet looked up by UUID, and snapshotFind
looked up by name; after that point, snapshotGet disappeared
and snapshotFind handles uuid or name.

* src/vbox/vbox_tmpl.c (vboxDomainSnapshotNum)
(vboxDomainSnapshotListNames): Implement limiting list to root.