Make block pull event dispatcher private

The method dispatchDomainEventBlockPullCallback which is
used internally to dispatch block pull events to the python
application code was missing the leading '_', to denote that
it was private.  All other event callback helpers have a
leading '_'. No application should have been using this so
it is justifiable to rename it.

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

libvirt-override.c

@@ -5998,7 +5998,7 @@ libvirt_virConnectDomainEventBlockJobCallback(virConnectPtr conn ATTRIBUTE_UNUSE
 
     /* Call the Callback Dispatcher */
     pyobj_ret = PyObject_CallMethod(pyobj_conn,
-                                    (char*)"dispatchDomainEventBlockPullCallback",
+                                    (char*)"_dispatchDomainEventBlockPullCallback",
                                     (char*)"OsiiO",
                                     pyobj_dom, path, type, status, pyobj_cbData);