numa: reduce code duplication by adding helper numa_get_node_for_cpu()

Replace repeated pattern

    for (i = 0; i < nb_numa_nodes; i++) {
        if (test_bit(idx, numa_info[i].node_cpu)) {
           ...
           break;

with a helper function to lookup numa node index for cpu.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>