health_libs: Fixing broken fix ;o)

A previous fix did broke this code and prevented it to run properly.

Error look like :
Benchmarking all CPUs for 10 seconds (2 threads)
Traceback (most recent call last):
  File "./health-client.py", line 174, in <module>
    connect_to_server(hrdw, sys.argv[2])
  File "./health-client.py", line 155, in connect_to_server
    handlers[msg.message](s, msg)
  File "./health-client.py", line 110, in module
    handlers[msg.module](socket, msg)
  File "./health-client.py", line 71, in cpu
    action(socket, msg, HCPU(msg, socket, HP.logger))
  File "./health-client.py", line 97, in action
    handlers[msg.action]()
  File "/health_bench.py", line 63, in start
    self.message.cpu_instances)
  File "/health_libs.py", line 42, in run_sysbench
    (taskset, max_time, cpu_count)