simpletrace: Convert name from mapping record to str

simpletrace: Convert name from mapping record to str

The rest of the code assumes that idtoname is a (int -> str)
dictionary, so convert the data accordingly.

This is necessary to make the script work with Python 3 (where
reads from a binary file return 'bytes' objects, not 'str').

Fixes the following error:

  $ python3 ./scripts/simpletrace.py trace-events-all trace-27445
  b'object_class_dynamic_cast_assert' event is logged but is not \
  declared in the trace events file, try using trace-events-all instead.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-id: 20180619194549.15584-1-ehabkost@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>