libvirt: Fix schedinfo cgroup bug

This PR depends on #1253 

This fix use 'open' instead of 'cat' to access cgroup file to prevent
problem if slash ('\') exist in file path.

If the cgroup path has a 'emulator' sub-directory. We need to trim it to
get the desired cgroup file.

Signed-off-by: Hao Liu <hliu@redhat.com>