Add nx test case

Sync nc test case to upstream.

    qemu: Add a subtest nx.
    
    Test whether nx(cpu) bit takes effect in guest.
    
    Signed-off-by: Ken Cao <kcao@redhat.com>
    Signed-off-by: Feng Yang <fyang@redhat.com>
    KVM test: Rename the nx_exploit.py to nx.py
    Signed-off-by: Jason Wang <jasowang@redhat.com>
    Reviewed-by: Yolkfull Chow <yzhou@redhat.com>