conf: add caching_mode attribute to iommu device

Add a new attribute to control the caching mode.

https://bugzilla.redhat.com/show_bug.cgi?id=1427005

docs/formatdomain.html.in

@@ -7427,6 +7427,15 @@ qemu-kvm -net nic,model=? /dev/null
               <span class="since">Since 3.4.0</span> (QEMU/KVM only)
             </p>
           </dd>
+          <dt><code>caching_mode</code></dt>
+          <dd>
+            <p>
+              The <code>caching_mode</code> attribute with possible values
+              <code>on</code> and <code>off</code> can be used to
+              turn on the VT-d caching mode (useful for assigned devices).
+              <span class="since">Since 3.4.0</span> (QEMU/KVM only)
+            </p>
+          </dd>
         </dl>
       </dd>
     </dl>

docs/schemas/domaincommon.rng

@@ -3954,6 +3954,11 @@
               <ref name="virOnOff"/>
             </attribute>
           </optional>
+          <optional>
+            <attribute name="caching_mode">
+              <ref name="virOnOff"/>
+            </attribute>
+          </optional>
         </element>
       </optional>
     </element>