Create a new file for managing a list of checkpoint objects, borrowing
heavily from existing virDomainSnapshotObjList paradigms.
Note that while snapshots definitely have a use case for multiple
children to a single parent (create a base snapshot, create a child
snapshot, revert to the base, then create another child snapshot),
it's harder to predict how checkpoints will play out with reverting to
prior points in time. Thus, in initial use, given a list of
checkpoints, you never have more than one child, and we can treat the
most-recent leaf node as the parent of the next node creation, without
having to expose a notion of a current node in XML or public API.
However, as the snapshot machinery is already generic, it is easier to
reuse the generic machinery that tracks relations between domain
moments than it is to open-code a new list-management scheme just for
checkpoints (hence, we still have internal functions related to a
current checkpoint, even though that has no observable effect
externally, as well as the addition of a function to easily find the
lone leaf in the list to use as the current checkpoint).
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
