Merge branch 'stable-2.15' into stable-2.16

* stable-2.15
  Cleanup more pylint/pep8/apidoc errors
  KVM: handle gracefully too old/too new psutil versions

Manually fix up merge conflicts.

Signed-off-by: Brian Foley <bpfoley@google.com>
Reviewed-by: Viktor Bachraty <vbachraty@google.com>