By using the more specific type, we get fewer downcasts.  The
downcasts are safe, but not obviously so, at least not locally.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <20180703085358.13941-24-armbru@redhat.com>