Allow to skip cpu_mode for virt node

PROD-36510

Change-Id: I395a99eb2794a5b14dda68d8ed528f0c2905f508
diff --git a/salt/control/virt.sls b/salt/control/virt.sls
index 604bc3d..7751e6e 100644
--- a/salt/control/virt.sls
+++ b/salt/control/virt.sls
@@ -90,7 +90,7 @@
   {%- if node.cpuset is defined %}
   - cpuset: {{ node.cpuset }}
   {%- endif %}
-  {%- if node.cpu_mode is defined %}
+  {%- if node.get('cpu_mode') not in [None, '', 'qemu_default'] %}
   - cpu_mode: {{ node.cpu_mode }}
   {%- endif %}
   - kwargs: