vpp_papi: Fix vapi.disconnect exception.

vpp_papi: Fix vapi.disconnect exception.

Traceback (most recent call last):
  File "/vpp/test/framework.py", line 521, in setUpClass
    cls.quit()
  File "/vpp/test/framework.py", line 556, in quit
    cls.vapi.disconnect()
  File "/vpp/test/vpp_papi_provider.py", line 308, in disconnect
    self.vpp.disconnect()
  File "/vpp/src/vpp-api/python/vpp_papi/vpp_papi.py", line 500, in disconnect
    if self.event_thread is not None:
AttributeError: 'VPPApiClient' object has no attribute 'event_thread'

Type: fix

Change-Id: I7a8db08b0922bf92ab42df121b1707073af9cedf
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>