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).