Add virtio-related options to interfaces

<interface type='user'>
  <mac address='52:54:56:5a:5c:5e'/>
  <model type='virtio'/>
  <driver iommu='on' ats='on'/>
</interface>

https://bugzilla.redhat.com/show_bug.cgi?id=1283251

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>

docs/formatdomain.html.in

@@ -3451,6 +3451,19 @@
       </dd>
     </dl>
 
+    <h4><a name="elementsVirtio">Virtio-related options</a></h4>
+
+    <p>
+      QEMU's virtio devices have some attributes related to the virtio transport under
+      the <code>driver</code> element:
+      The <code>iommu</code> attribute enables the use of emulated IOMMU
+      by the device. The attribute <code>ats</code> controls the Address
+      Translation Service support for PCIe devices. This is needed to make use
+      of IOTLB support (see <a href="#elementsIommu">IOMMU device</a>).
+      Possible values are <code>on</code> or <code>off</code>.
+      <span class="since">Since 3.5.0</span>
+    </p>
+
     <h4><a name="elementsControllers">Controllers</a></h4>
 
     <p>
@@ -5142,6 +5155,12 @@ 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>virtio options</dt>
+      <dd>
+        For virtio interfaces,
+        <a href="#elementsVirtio">Virtio-specific options</a> can also be
+        set. (<span class="since">Since 3.5.0</span>)
+      </dd>
     </dl>
     <p>
       Offloading options for the host and guest can be configured using

docs/schemas/domaincommon.rng

@@ -2686,6 +2686,7 @@
               </optional>
             </group>
           </choice>
+          <ref name="virtioOptions"/>
           <interleave>
             <optional>
               <element name='host'>
@@ -5006,6 +5007,17 @@
     </element>
   </define>
 
+  <define name="virtioOptions">
+    <optional>
+      <attribute name="iommu">
+        <ref name="virOnOff"/>
+      </attribute>
+      <attribute name="ats">
+        <ref name="virOnOff"/>
+      </attribute>
+    </optional>
+  </define>
+
   <define name="usbmaster">
     <element name="master">
       <attribute name="startport">