qa/common.check: improvements for pmlogger and pmie tests
qa/common.check: improvements for pmlogger and pmie tests

- add _wait_for_pmie() and _wait_pmie_end() to detect started and
  finished conditions for the primary pmie ... needed to avoid race
  conditions with the rc/systemd starting and stopping of pmie
- harden up _disable_loggers() and _restore_loggers() to reduce the
  chance of key config files being trashed by errant QA tests.
  Note these functions do not really disable pmloggers, rather they
  allow a dormant primary pmlogger to be started with no metrics to
  be logged, so pmlogger is not talking to pmcd ... this is only
  needed/used in a few QA tests.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp