lisa._assets.kmodules.lisa: Get task_group sched_avg for *_cfs_tg events

FIX

_cfs_tg events will now use the task_group->se[cpu]->avg sched_avg instead of the
cfs_rq->avg.

This allows consistent logging with the "real" task group sched_avg, so
with that change it makes sense to add a probe on pelt_se_tp as well to
track the fine-grained evolutions of that value.