This introduces four new options for secret-list, to filter the
returned secrets by whether it's ephemeral or not, and/or by
whether it's private or not.

* tools/virsh-secret.c: (New helper vshSecretSorter,
  vshSecretListFree, and vshCollectSecretList; Use the new
  API for secret-list; error out if flags are specified,
  because there is no way to filter the results when using
  old APIs (no APIs to get the properties (ephemeral, private)
  of a secret yet).

* tools/virsh.pod: Document the 4 new options.