Introduce /domain/devices/interface/driver/@queues attribute

This attribute is going to represent number of queues for
multique vhost network interface. This commit implements XML
extension part of the feature and add one test as well. For now,
we can only do xml2xml test as qemu command line generation code
is not adapted yet.

docs/formatdomain.html.in

@@ -3265,7 +3265,7 @@ qemu-kvm -net nic,model=? /dev/null
       <source network='default'/>
       <target dev='vnet1'/>
       <model type='virtio'/>
-      <b>&lt;driver name='vhost' txmode='iothread' ioeventfd='on' event_idx='off'/&gt;</b>
+      <b>&lt;driver name='vhost' txmode='iothread' ioeventfd='on' event_idx='off' queues='5'/&gt;</b>
     &lt;/interface&gt;
   &lt;/devices&gt;
   ...</pre>
@@ -3359,6 +3359,16 @@ qemu-kvm -net nic,model=? /dev/null
         <b>In general you should leave this option alone, unless you
         are very certain you know what you are doing.</b>
       </dd>
+      <dt><code>queues</code></dt>
+      <dd>
+        The optional <code>queues</code> attribute controls the number of
+        queues to be used for the<a href="http://www.linux-kvm.org/page/Multiqueue">
+        Multiqueue virtio-net</a> feature. If the interface has <code>&lt;model
+        type='virtio'/&gt;</code>, multiple packet processing queues can be
+        created; each queue will potentially be handled by a different
+        processor, resulting in much higher throughput.
+        <span class="since">Since 1.0.6 (QEMU and KVM only)</span>
+      </dd>
     </dl>
 
     <h5><a name="elementsNICSTargetOverride">Overriding the target element</a></h5>

docs/schemas/domaincommon.rng

@@ -2012,6 +2012,11 @@
                   </choice>
                 </attribute>
               </optional>
+              <optional>
+                <attribute name='queues'>
+                  <ref name="positiveInteger"/>
+                </attribute>
+              </optional>
               <optional>
                 <attribute name="txmode">
                   <choice>