snapshot: Access snapshot def directly when needed

snapshot: Access snapshot def directly when needed

An upcoming patch will rework virDomainSnapshotObjList to be generic
for both snapshots and checkpoints; reduce the churn by adding a new
accessor virDomainSnapshotObjGetDef() which returns the
snapshot-specific definition even when the list is rewritten to
operate only on a base class, then using it at sites that that are
specific to snapshots.  Use VIR_STEAL_PTR when appropriate in the
affected lines.

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