selftests/resctrl: Consolidate naming of perf event related things

Naming for perf event related functions, types, and variables is
inconsistent.

Make struct read_format and all functions related to perf events start
with "perf_". Adjust variable names towards the same direction but use
shorter names for variables where appropriate (pe prefix).

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>