In preparation for making virDomainSnapshotDef a descendant of
virObject, it is time to fix all callers that allocate an object to
use virDomainSnapshotDefNew() instead of VIR_ALLOC().  Fortunately,
there aren't very many :)
Signed-off-by: NEric Blake <eblake@redhat.com>
Acked-by: NPeter Krempa <pkrempa@redhat.com>