chdir() to correct path after calling chroot()

Since chroot() doesn't chdir() by default, we are left in
an unreachable directory in node.pexec() (and in xterms.)

fixes #370
use cgroups to calculate percentage of cpu used
Accept 'ISO' or 'boot' in file *.iso command output

Fixes #372
qcow2size(): use qemu-image instead of file

file no longer returns image size on 14.04
fixes #373
Merge pull request #369 from cdburkard/patches/hifi_multicore

Fix runCpuLimitTest with multiple cores