perf scripts python: exported-sql-viewer.py: Add top calls report

Add a new report to display top calls by elapsed time. It displays calls
in descending order of time elapsed between when the function was called
and when it returned.

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