We provide the virEventRegister() API to let applications provide
a set of callbacks to integrate with their existing event loop.
Not all apps have an existing event loop and forcing them to write
one themselves is an undue burden, which they often get wrong. We
have an event loop impl used by libvirtd that could be used by
applications. This series exposes it via two new public APIs. Apps
are free to ignore these new APIs and provide their own impl, but
this should help the common case.