virnetdaemon: Don't deadlock when talking to D-Bus
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1487322
    
    In ace45e67abbd I tried to fix a problem that we get the reply to
    a D-Bus call while we were sleeping. In that case the callback
    was never set. So I changed the code that the callback is called
    directly in this case. However, I hadn't realized that since the
    callback is called out of order it locks the virNetDaemon.
    Exactly the very same virNetDaemon object that we are dealing
    with right now and that we have locked already (in
    virNetDaemonAddShutdownInhibition())
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>