libvirt-test: Fix cpu-stats cgroup problem

* This fix use utils_cgroup to get cgroup path instead of using
  hard-coded path which causes compatibility problem.

* Built in python module is used to get processors' count instead
  of hard-coded command line.

* These tests is still failing because of a bug of return values.

Signed-off-by: Hao Liu <hliu@redhat.com>