lib/group_cpus: Export group_cpus_evenly()

Export group_cpus_evenly() so that some modules
can make use of it to group CPUs evenly according
to NUMA and CPU locality.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230323053043.35-2-xieyongji@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>