qemu.tests.cfg: Add config file for nx.py script.

Signed-off-by: NFeng Yang <fyang@redhat.com>
Signed-off-by: NCao, Chen <kcao@redhat.com>
KVM-TEST: use cfg to store the ", " on cpu flags
Signed-off-by: NXiaoqing Wei <xwei@redhat.com>

qemu/tests/cfg/nx.cfg

+- nx:
+    only Linux
+    no RHEL.4
+    only x86_64
+    type = nx
+    virt_test_type = qemu
+    image_snapshot = yes
+    kill_vm = yes
+    variants:
+        - noexec_on:
+            nx_on = yes
+        - noexec_off:
+            cpu_model_flags = ",-nx"
+            nx_on = no