cirrus_vga: adding sanity check for vram size

when configure a invalid vram size for cirrus card, such as less
2 MB, which will crash qemu. Follow the real hardware, the cirrus
card has 4 MB video memory. Also for backward compatibility, accept
8 MB and 16 MB vram size.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>