Performance graph traceback if xenserver tools not installed

[TwoWheelDev]

Exception in thread Thread-11:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 552, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 505, in run
    self.__target(*self.__args, **self.__kwargs)
  File "./src/OXM/oxcSERVER.py", line 717, in update_performance
    chart["vbd"].set_size_request(len(data)*20, 250)
TypeError: object of type 'NoneType' has no len()