Ensure we return the callback ID in python events binding

A missing return statement in the python binding meant that
the callers could not get the callback ID, and thus not be
able to unregister event callbacks

* python/libvirt-override-virConnect.py: Add missing return
  statement

python/libvirt-override-virConnect.py

@@ -146,3 +146,4 @@
         if ret == -1:
             raise libvirtError ('virConnectDomainEventRegisterAny() failed', conn=self)
         self.domainEventCallbackID[ret] = opaque
+        return ret