-1000 was used as a special value added in Commit 3d5045492ab2 ("perf
pmu-events: Add pmu_events_table__find_event()") to show that 1 table
lacked a PMU/event but that didn't terminate the search in other
tables. Add a new constant PMU_EVENTS__NOT_FOUND for this value and
use it.
Signed-off-by: Ian Rogers<irogers@xxxxxxxxxx>