docs: domain: Show canonical pvspinlock XML

The example pvspinlock XML is:

  <pvspinlock/>

While this is accepted by libvirt and works correctly, it's currently
always output as a tristate like

  <pvspinlock state='on'/>

So document that format instead

docs/formatdomain.html.in

@@ -1464,7 +1464,7 @@
     <kvm>
       <hidden state='on'/>
     </kvm>
-    <pvspinlock/>
+    <pvspinlock state='on'/>
     <gic version='2'/>
 
   </features>