libxl: fix cirrus vga video memory setting with upstream qemu

Reading one qemu-devel post seems that setting video memory of
cirrus vga with upstream qemu is wrong even if not show errors.

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>

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Note:
- when this patch will be accepted upstream should be
  backported also on xen 4.4
- with this qemu parameters seems correct but for further
  confirmation I posted a question about it:
http://lists.xen.org/archives/html/xen-devel/2014-04/msg02606.html