events: make the callback registry a bit more generic
events: make the callback registry a bit more generic

We want to be able to register different kind of callbacks within the
registry, not just domain callbacks. Therefore, we make the registry a
bit more generic. Currently, there is no functionnality change.

libvirt/libvirt-go,rgbkrk/libvirt-go,libvirt/libvirt-go,rgbkrk/libvirt-go,alexzorin/libvirt-go,libvirt/libvirt-go,Pryz/libvirt-go,rgbkrk/libvirt-go,alexzorin/libvirt-go,libvirt/libvirt-go,Pryz/libvirt-go,Pryz/libvirt-go