perf evswitch: Introduce init() method to set the on/off evsels from the command line

Another step in having all the boilerplate in just one place to then use
in the other tools.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: William Cohen <wcohen@redhat.com>
Link: https://lkml.kernel.org/n/tip-snreb1wmwyjei3eefwotxp1l@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>