perf stat: Introduce read_counters function

perf stat: Introduce read_counters function

Moving read counters logic into single read_counters function, which
will be called for both interval and overall processing legs.

The reason is to split reading and processing (following patches)
counters code, so we could read counters from other sources (like
perf.data) and process them in the same way as 'perf stat' command does.

Signed-off-by: Jiri Olsa <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>