procstats.py: record per CPU values from /proc/stat

Updating procstats.py to use per-cpu breakdowns (cpu0, cpu1, etc)
instead of the aggregated "cpu" line. This works even in the case of
single-core boxes, which have identical cpu and cpu0 lines.

Also, I corrected the tag names for columns 8 and 9 to steal and guest as per proc(5).
procstats.py Use only the CPU number as value for the cpu tag

cpu=X instead of cpu=cpuX