snapshot: Make virDomainSnapshotObjList use MomentObjList

snapshot: Make virDomainSnapshotObjList use MomentObjList

Now that the generic moment code does pretty much everything that both
snapshots and checkpoints will need, it's time to replace the
now-duplicate code in virdomainsnapshotobjlist.c with simpler calls
into the generic code. I considered using sub-classing (a
'virDomainMomentObjList parent;' member, but that requires making the
opaque type visible in headers; so for now, I stuck with a container
instead (a 'virDomainMomentObjListPtr base;' member).

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