MNAIO: Use virt module to shut VM's down when saving images

It's better to shut the VM's down cleanly instead of just turning
them off, so we change 'destroy' to 'shutdown' and use the virt
module for this action instead of the command module.

Change-Id: I896b7794328b91dc59726bf1d5366eeb7112ca21

multi-node-aio/playbooks/save-vms.yml

@@ -37,8 +37,9 @@
       register: _virt_pools
 
     - name: Stop running VMs
-      command: "virsh destroy {{ hostvars[item]['server_hostname'] }}"
-      failed_when: false
+      virt:
+        name: "{{ hostvars[item]['server_hostname'] }}"
+        state: shutdown
       when:
         - hostvars[item]['server_vm'] | default(false) | bool
       with_items: "{{ groups['pxe_servers'] }}"