qxl: use uint64_t for vram size

This allows for the 64bit vram bar to become larger than 2G
(try -device qxl-vga,vram64_size_mb=8192).

https://bugzilla.redhat.com/show_bug.cgi?id=1340439

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1465389648-5179-1-git-send-email-kraxel@redhat.com
