Fix and improve domain xml video element description

The description mismatched the actual structure since the video element
was introduced. The nvram attribute is actually called vram. Specify the
unit of the vram attribute.

docs/formatdomain.html.in

@@ -774,20 +774,34 @@ qemu-kvm -net nic,model=? /dev/null
 
     <pre>
       ...
-      &lt;video type='vga' nvram='8192' heads='1'&gt;
-        &lt;acceleration accel3d='yes' accel3d='yes' /&gt;
+      &lt;video&gt;
+        &lt;model type='vga' vram='8192' heads='1'&gt;
+          &lt;acceleration accel3d='yes' accel3d='yes'/&gt;
+        &lt;/model&gt;
       &lt;/video&gt;
       ...</pre>
 
     <dl>
       <dt><code>video</code></dt>
-      <dd>The <code>video</code> element has a mandatory <code>type</code>
-  attribute which takes the value "vga", "cirrus", "vmvga", "xen" or "vbox".
-  You can also provide the amount of video memory using <code>nvram</code>,
-  the number of screen with <code>heads</code>, and whether acceleration
-  should be enabled (if supported) using the <code>accel3d</code> and
-  <code>accel2d</code> attributes in the <code>acceleration</code> element.
+      <dd>
+        The <code>video</code> element is the a container for describing
+        video devices.
+      </dd>
+
+      <dt><code>model</code></dt>
+      <dd>
+        The <code>model</code> element has a mandatory <code>type</code>
+        attribute which takes the value "vga", "cirrus", "vmvga", "xen" or "vbox".
+        You can also provide the amount of video memory in kilobytes using
+        <code>vram</code> and the number of screen with <code>heads</code>.
+      </dd>
 
+      <dt><code>acceleration</code></dt>
+      <dd>
+        If acceleration should be enabled (if supported) using the
+        <code>accel3d</code> and <code>accel2d</code> attributes in the
+        <code>acceleration</code> element.
+      </dd>
     </dl>
 
     <h4><a name="elementsConsole">Consoles, serial, parallel &amp; channel devices</a></h4>