simpletrace: Improve the error message if event is not declared

simpletrace: Improve the error message if event is not declared

Today, if we use a trace-event file which does not declare an event
existing in the log file we'll get the following error:

$ scripts/simpletrace.py trace-events trace-68508
Traceback (most recent call last):
  File "scripts/simpletrace.py", line 242, in <module>
    run(Formatter())
  File "scripts/simpletrace.py", line 217, in run
    process(events, sys.argv[2], analyzer, read_header=read_header)
  File "scripts/simpletrace.py", line 192, in process
    for rec in read_trace_records(edict, log):
  File "scripts/simpletrace.py", line 107, in read_trace_records
    rec = read_record(edict, idtoname, fobj)
  File "scripts/simpletrace.py", line 71, in read_record
    return get_record(edict, idtoname, rechdr, fobj)
  File "scripts/simpletrace.py", line 45, in get_record
    event = edict[name]
KeyError: 'qemu_mutex_locked'

This patch improves this error by adding a hint instead of just that
KeyError log:

$ scripts/simpletrace.py trace-events trace-68508
'qemu_mutex_locked' event is logged but is not declared in the trace
events file, try using trace-events-all instead.

Signed-off-by: Jose Ricardo Ziviani <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: 1496075404[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>