ioemu/qemu vga: save and restore vram buffer (take 2)

ioemu/qemu vga: save and restore vram buffer (take 2)

The existing stdvga driver from xen-unstable tools/ioemu/hw/vga* does
not save the emulated VGA memory contents.  The symptoms include video
malfunction after restore, including black screen (which can often be
fixed by asking the guest to redraw) but also missing font setup etc.
The attached patch fixes this by saving the entire VGA memory buffer,
just like the Xen ioemu Cirrus emulator does.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>