snapshot: Create new virDomainMomentObjList type

snapshot: Create new virDomainMomentObjList type

The new code here very heavily resembles the code in
virDomainSnapshotObjList. There are still a couple of spots that are
tied a little too heavily to snapshots (the free function lacks a
polymorphic cleanup until we refactor code to use virObject; and an
upcoming patch will add internal VIR_DOMAIN_MOMENT_LIST flags to
replace the snapshot flag bits), but in general this is fairly close
to the state needed to add checkpoint lists.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>