snapshot: Switch type of virDomainSnapshotObj.def

snapshot: Switch type of virDomainSnapshotObj.def

Another step towards making the object list reusable for both
snapshots and checkpoints: the list code only ever needs items that
are in the common virDomainMomentDef base type. This undoes a lot of
the churn in accessing common members added in the previous patch, and
the bulk of the patch is mechanical. But there was one spot where I
had to unroll a VIR_STEAL_PTR to work around changed types.

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