conf: support backend domain name in disk and network devices

At least Xen supports backend drivers in another domain (aka "driver
domain"). This patch introduces an XML config option for specifying the
backend domain name for <disk> and <interface> devices.  E.g.

  <disk>
    <backenddomain name='diskvm'/>
    ...
  </disk>
  <interface type='bridge'>
    <backenddomain name='netvm'/>
    ...
  </interface>

In the future, same option will be needed for USB devices (hostdev
objects), but for now libxl doesn't have support for PVUSB.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

docs/formatdomain.html.in

@@ -2372,6 +2372,12 @@
           </li>
         </ul>
       </dd>
+      <dt><code>backenddomain</code></dt>
+      <dd>The optional <code>backenddomain</code> element allows specifying a
+          backend domain (aka driver domain) hosting the disk.  Use the
+          <code>name</code> attribute to specify the backend domain name.
+          <span class="since">Since 1.2.13 (Xen only)</span>
+      </dd>
       <dt><code>boot</code></dt>
       <dd>Specifies that the disk is bootable. The <code>order</code>
         attribute determines the order in which devices will be tried during
@@ -4256,6 +4262,29 @@ qemu-kvm -net nic,model=? /dev/null
       network device.
       <span class="since">Since 0.9.10 (QEMU and KVM only)</span>.
     </p>
+    <h5><a name="elementDomain">Setting up a network backend in a driver domain</a></h5>
+<pre>
+  ...
+  &lt;devices&gt;
+    ...
+    &lt;interface type='bridge'&gt;
+      &lt;source bridge='br0'/&gt;
+      <b>&lt;backenddomain name='netvm'/&gt;</b>
+    &lt;/interface&gt;
+    ...
+  &lt;/devices&gt;
+  ...</pre>
+
+    <p>
+      The optional <code>backenddomain</code> element allows specifying a
+      backend domain (aka driver domain) for the interface. Use the
+      <code>name</code> attribute to specify the backend domain name. You
+      can use it to create a direct network link between domains (so data
+      will not go through host system). Use with type 'ethernet' to create
+      plain network link, or with type 'bridge' to connect to a bridge inside
+      the backend domain.
+      <span class="since">Since 1.2.13 (Xen only)</span>
+    </p>
 
     <h5><a name="elementQoS">Quality of service</a></h5>
 

docs/schemas/domaincommon.rng

@@ -1173,6 +1173,14 @@
       <optional>
         <ref name="deviceBoot"/>
       </optional>
+      <optional>
+        <element name="backenddomain">
+          <attribute name="name">
+            <ref name="domainName"/>
+          </attribute>
+          <empty/>
+        </element>
+      </optional>
       <optional>
         <element name="readonly">
           <empty/>
@@ -2371,6 +2379,14 @@
           <empty/>
         </element>
       </optional>
+      <optional>
+        <element name="backenddomain">
+          <attribute name="name">
+            <ref name="domainName"/>
+          </attribute>
+          <empty/>
+        </element>
+      </optional>
       <optional>
         <element name="model">
           <attribute name="type">