virt: Modify test-example for mathine type

Since machine_type 'q35' has been added to machine.cfg.If I do not change tests.cfg, when I execute libvirt a test case , I will get two:
```javascript
16:36:04 INFO | Test    1:  qcow2.virtio_blk.smp2.virtio_net.RHEL.6.0.x86_64.virsh_help.normal_test
16:36:04 INFO | Test    2:  qcow2.virtio_blk.smp2.virtio_net.RHEL.6.0.x86_64.q35.virsh_help.normal_test
```
So I think perhaps tests-example.cfg need a modification that show users how to run one case.