Introduce <iommu> device

A device with an attribute 'model', with just one model
so far:

<devices>
  ...
  <iommu model='intel'/>
</devices>

https://bugzilla.redhat.com/show_bug.cgi?id=1235580

docs/formatdomain.html.in

@@ -6752,6 +6752,32 @@ qemu-kvm -net nic,model=? /dev/null
       </dd>
     </dl>
 
+    <h4><a name="elementsIommu">IOMMU devices</a></h4>
+
+    <p>
+      The <code>iommu</code> element can be used to add an IOMMU device.
+      <span class="since">Since 2.1.0</span>
+    </p>
+
+    <p>
+      Example:
+    </p>
+<pre>
+  ...
+  &lt;devices&gt;
+    &lt;iommu model='intel'/&gt;
+  &lt;/devices&gt;
+  ...
+</pre>
+    <dl>
+      <dt><code>model</code></dt>
+      <dd>
+        <p>
+          Currently only the <code>intel</code> model is supported.
+        </p>
+      </dd>
+    </dl>
+
     <h3><a name="seclabel">Security label</a></h3>
 
     <p>

docs/schemas/domaincommon.rng

@@ -3718,6 +3718,14 @@
    </optional>
   </define>
 
+  <define name="iommu">
+    <element name="iommu">
+      <attribute name="model">
+        <value>intel</value>
+      </attribute>
+    </element>
+  </define>
+
   <define name="input">
     <element name="input">
       <choice>
@@ -4187,6 +4195,9 @@
         <zeroOrMore>
           <ref name="panic"/>
         </zeroOrMore>
+        <optional>
+          <ref name="iommu"/>
+        </optional>
       </interleave>
     </element>
   </define>