docs: event impl. registration before hypervisor connection
    
    Event implementations need to be registered before a connection to the
    Hypervisor is opened, otherwise event handling can be impaired (e.g.
    delayed messages). This fact is referenced in an e-mail [1], but should
    also be noted in the documentation of the registration functions.
    
    [1] https://www.redhat.com/archives/libvirt-users/2014-April/msg00011.html