iotests: remove assertIsNotNone call

RHEL6 doesn't have Python 2.7, so replace this call with
assertNotEqual(x, None) which will work just as well.
Reported-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/124

@@ -125,7 +125,7 @@ class TestIncrementalBackup(iotests.QMPTestCase):
 
         event = self.vm.event_wait(name="BLOCK_JOB_COMPLETED",
                                    match={'data': {'device': kwargs['device']}})
-        self.assertIsNotNone(event)
+        self.assertNotEqual(event, None)
 
         try:
             failure = self.dictpath(event, 'data/error')