perfcounters: use hw_event.disable flag

Impact: implement default-off counters

Make sure that counters that are created with counter.hw_event.disabled=1,
get created in disabled state.

They can be enabled via:

        prctl(PR_TASK_PERF_COUNTERS_ENABLE);

Signed-off-by: Ingo Molnar <mingo@elte.hu>