Create a new child flow from a hunt.

def CallFlow(self,
               flow_name=None,
               next_state=None,
               request_data=None,
               client_id=None,
               **kwargs):
    """Create a new child flow from a hunt."""
    base_session_id = None
    if client_id:
      # The flow is stored in the hunt namespace,
      base_session_id = self.urn.Add(client_id.Basename())

    # Actually start the new flow.
    child_urn = self.runner.CallFlow(
        flow_name=flow_name,
        next_state=next_state,
        base_session_id=base_session_id,
        client_id=client_id,
        request_data=request_data,
        **kwargs)

    if client_id:
      # But we also create a symlink to it from the client's namespace.
      hunt_link_urn = client_id.Add("flows").Add("%s:hunt" %
                                                 (self.urn.Basename()))

      hunt_link = aff4.FACTORY.Create(
          hunt_link_urn, aff4.AFF4Symlink, token=self.token)

      hunt_link.Set(hunt_link.Schema.SYMLINK_TARGET(child_urn))
      hunt_link.Close()

    return child_urn