sst: Prefer cached values over default values

In anticipation of support for future CPUs, change some assumptions:
* When host is off, return cached property values instead of "default"
  values. CPUs may not guarantee support for level 0, so there is no
  universal default.
* Must always check the backend interface is ready before using it.
* Rename numLevels() to maxLevel() since there may be discontinuities in
  the supported levels.
* Also add some more debug prints.

Tested:
* On CPU that supports level 0 - verified that output of sst-info.sh
  script was the same before and after these changes.
* On a CPU that only supports level 4 - verified that when host was
  powered off, the AppliedConfig showed config4 (valid) instead of
  config0 (invalid).

Change-Id: Idffcb9a6534ba32760f6d6b2ac244f47427995ea
Signed-off-by: Jonathan Doman <jonathan.doman@intel.com>
3 files changed