Fix cpu checks for older Linux kernels

Older Linux kernels don't have the "guest" field in /proc/stat.  While
check-cpu.rb claimed to accommodate that, it didn't actually work all
the way through.  Fix that, and also fix cpu-pcnt-usage-metrics.rb,
which didn't know about this at all.
Merge pull request #423 from petere/cpu-without-guest

Fix cpu checks for older Linux kernels