Fix virCgroupGetPercpuStats with non-continuous present CPUs

Fix virCgroupGetPercpuStats with non-continuous present CPUs

Per-cpu stats are only shown for present CPUs in the cgroups,
but we were only parsing the largest CPU number from
/sys/devices/system/cpu/present and looking for stats even for
non-present CPUs.
This resulted in:
internal error: cpuacct parse error