qemu: Add support for paravirtual spinlocks in the guest

The linux kernel recently added support for paravirtual spinlock
handling to avoid performance regressions on overcomitted hosts. This
feature needs to be turned in the hypervisor so that the guest OS is
notified about the possible support.

This patch adds a new feature "paravirt-spinlock" to the XML and
supporting code to enable the "kvm_pv_unhalt" pseudo CPU feature in
qemu.

https://bugzilla.redhat.com/show_bug.cgi?id=1008989

docs/formatdomain.html.in

@@ -1193,6 +1193,7 @@
       <vapic state='on'/>
       <spinlocks state='on' retries='4096'/>
     </hyperv>
+    <pvspinlock/>
 
   </features>
   ...</pre>
@@ -1264,6 +1265,13 @@
         </tr>
       </table>
       </dd>
+      <dt><code>pvspinlock</code></dt>
+      <dd>Notify the guest that the host supports paravirtual spinlocks
+          for example by exposing the pvticketlocks mechanism. This feature
+          can be explicitly disabled by using <code>state='off'</code>
+          attribute.
+      </dd>
+
     </dl>
 
     <h3><a name="elementsTime">Time keeping</a></h3>