[telemetry] Reland MSR power monitor.

[telemetry] Reland MSR power monitor.

New! With:
- Disable for platforms that report no data.
- Extra logging to help diagnose why it's disabled.
- Fixes asserts (assert _var is not None instead of just _var).


BUG=336558
TEST=python tools/telemetry/run_tests msr
R=tonyg

Committed: https://crrev.com/05905bc9e13ee53917bf5edaa9741f48672180e7
Cr-Commit-Position: refs/heads/master@{#294084}

[tonyg, 2014-09-10 01:45:57]

lgtm

[dtu, 2014-09-10 01:58:05]

The CQ bit was checked by dtu@chromium.org

[commit-bot: I haz the power, 2014-09-10 02:00:03]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/dtu@chromium.org/553163004/1

[commit-bot: I haz the power, 2014-09-10 03:06:03]

Committed patchset #1 (id:1) as 288da7a974756befe59cbde8666cc4812ba51d94

[commit-bot: I haz the power, 2014-09-10 03:58:33]

Patchset 1 (id:??) landed as
https://crrev.com/05905bc9e13ee53917bf5edaa9741f48672180e7
Cr-Commit-Position: refs/heads/master@{#294084}