pkg/cgroups: Export ParseCgroupFile

This is needed for the libvirt exec driver.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Initial version of libvirt execdriver

This is a work in progress, it works at least for trivial cases, but
doesn't support networking yet, and will need more work.

Also, the backend is only build for the dynamic binary, because of problems
linking statically to libvirt.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)