perf python: Correct pyrf_evsel__read for tool PMUs

perf python: Correct pyrf_evsel__read for tool PMUs

Tool PMUs assume that stat's process_counter_values is being used to
read the counters. Specifically they hold onto old values in
evsel->prev_raw_counts and give the cumulative count based off of this
value. Update pyrf_evsel__read to allocate counts and prev_raw_counts,
use evsel__read_counter rather than perf_evsel__read so tool PMUs are
read from not just perf_event_open events, make the returned
pyrf_counts_values contain the delta value rather than the cumulative
value.

Fixes: 739621f65702 ("perf python: Add evsel read method")
Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250710235126.1086011-12-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>