Use pdh function for processor counter math

Use PdhGetFormattedCounterValue to do the math on processor usage,
rather than a version that uses processes the raw data.

Also switch to using %Processor Time rather than %Idle Time.  Note that
once in a while %Processor Time can go >100, I believe it's related to
frequency shifting.

This updates the change by @Karlson2k in https://github.com/xbmc/xbmc/commit/bbcc1f8b7740e76306f7a75825d2470de28180c9

@Karlson2k any comments on this?  I'm not sure if there was a reason you were using the raw values rather than using the format routine?