qemu: Support ram bar size for qxl devices

Adds a "ram" attribute globally to the video.model element, that changes
the resulting qemu command line only if video.type == "qxl".

<video>
  <model type='qxl' ram='65536' vram='65536' heads='1'/>
</video>

That attribute gets a default value of 64*1024. The schema is unchanged
for other video element types.

The resulting qemu command line change is the addition of

-global qxl-vga.ram_size=<ram>*1024

or

-global qxl.ram_size=<ram>*1024

For the main and secondary qxl devices respectively.

The default for the qxl ram bar is 64*1024 kilobytes (the same as the
default qxl vram bar size).

docs/formatdomain.html.in

@@ -3565,7 +3565,11 @@ qemu-kvm -net nic,model=? /dev/null
         video device in domain xml is the primary one, but the optional
         attribute <code>primary</code> (<span class="since">since 1.0.2</span>)
         with value 'yes' can be used to mark the primary in cases of mutiple
-        video device. The non-primary must be type of "qxl".
+        video device. The non-primary must be type of "qxl". The optional
+        attribute <code>ram</code> (<span class="since">since
+        1.0.2</span>) is allowed for "qxl" type only and specifies
+        the size of the primary bar, while <code>vram</code> specifies the
+        secondary bar size.  If "ram" is not supplied a default value is used.
       </dd>
 
       <dt><code>model</code></dt>