Clean up virCgroupGetPercpuStats

Clean up virCgroupGetPercpuStats

The iterator is checked for being less than or equal to need_cpus.
The 'n' variable is incremented need_cpus + 1 times.

Simplify the computation of need_cpus and make its value one larger,
to let it be used instead of 'n' and compared without the equal sign
in loop conditions.

Just index the sum_cpu_time array instead of using a helper variable.

Start the loop at start_cpu instead of continuing for all lower values.