virNetSocketRemoveIOCallback: Be explicit about unref
    
    When going through the code I've notice that
    virNetSocketAddIOCallback() increases the reference counter of
    @socket. However, its counter part RemoveIOCallback does not. It took
    me a while to realize this disproportion. The AddIOCallback registers
    our own callback which eventually calls the desired callback and then
    unref the @sock. Yeah, a bit complicated but it works. So, lets note
    this hard learned fact in a comment in RemoveIOCallback().
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>