perf evlist: Move the sideband thread routines to separate object

To avoid dragging more stuff into the perf python binding in the
following csets.

Reported-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>