Last I checked, the meaning of "self-serving" hasn't changed. Self-serving serves oneself. If Hillary's actions served the party, they too cannot not be termed self serving for such acts benefit someone other than Hillary Clinton. When people do things that serve their own and others benefit/ends, we describe such actions are "mutually beneficial," not self-serving. Mutually beneficial is what aptly describes Mrs. Clinton's appearances; Hillary is surely benefitting somehow from them, but so are others. And when one does something that does not benefit oneself in any way shape or form, well, that's called acting "selflessly."