python: treat flags as default argument with value 0

The following four functions have not changed because default arguments
have to come after positional arguments. Changing them will break the
the binding APIs.

migrate(self, dconn, flags, dname, uri, bandwidth):
migrate2(self, dconn, dxml, flags, dname, uri, bandwidth):
migrateToURI(self, duri, flags, dname, bandwidth):
migrateToURI2(self, dconnuri, miguri, dxml, flags, dname, bandwidth):

python/libvirt-override-virDomainSnapshot.py

@@ -6,7 +6,7 @@
         """Get the domain that a snapshot was created for"""
         return self.domain()
 
-    def listAllChildren(self, flags):
+    def listAllChildren(self, flags=0):
         """List all child snapshots and returns a list of snapshot objects"""
         ret = libvirtmod.virDomainSnapshotListAllChildren(self._o, flags)
         if ret is None: