virsh.emulatorpin: fix the way we get a cgroup path

There is a proper method in virttest.staging.utils_cgroup called
"resolve_task_cgroup_path" which is the best way to resolve
a cgroup path for current vm and cgroup controller.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Merge pull request #1150 from Antique/virsh-emulatorpin

virsh.emulatorpin: fix the way we get a cgroup path