WIP: create_perf_json: Support 'FIXED' event codes

[edwarddavidbaker]

This pull request unsets EventCode for FIXED events. Previously for fixed events the EventCode was 0x00. The get() helper used to process JSON attributes drops 0x00 (returns None). Here we add an extra check to also set event_code to None if EventCode is FIXED.

Example FIXED event:

        {
          "EventCode": "FIXED",
          "UMask": "0x01",
          "EventName": "INST_RETIRED.ANY",
          "Counter": "Fixed counter 0",
          "CounterHTOff": "Fixed counter 0",
          <snip>
          "Offcore": "0",
          "Deprecated": "0"
        }

[kshiprab]

Things to consider: We need to have consistent solution to indicate if counter is FIXED or FREERUN or Programmable.
Currently for SNR for IIO unit we have added Countertype column which indicates it is FREERUN counter. For Fixed counter is there any reason we cannot use CounterType column? In your above example we have more than one attribute indicating its Fixed counter.
EventCode and Counter and CounterHTOff attributes. Tools can use different fields for consumption. We need to be consistent for data publication and for tools consumption.

[edwarddavidbaker]

Abandoning example WIP pull request.