qxl: add vgamem_size_mb and vgamem_size

In preperation for supporting a larger framebuffer for multiple monitors
on a single card, add a property to qxl vgamem_size_mb, and corresponding
byte sized vgamem_size, and use instead of VGA_RAM_SIZE.

[ kraxel: simplify property handling, add sanity checks ]
[ kraxel: fix mode copying ]

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
