Fix race leading to crash when setting up dbus watches

Fix race leading to crash when setting up dbus watches

Currently the virDBusAddWatch does

  virEventAddHandle(fd, flags,
                    virDBusWatchCallback,
                    watch, NULL);
  dbus_watch_set_data(watch, info, virDBusWatchFree);

Unfortunately this is racy - since the event loop is in a
different thread, the virDBusWatchCallback method may be
run before we get to calling dbus_watch_set_data. We must
reverse the order of these calls

See https://bugzilla.redhat.com/show_bug.cgi?id=885445

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>