vga: make vram size configurable

vga: make vram size configurable

Zap the global VGA_RAM_SIZE #define, make the vga ram size configurable
for standard vga and vmware vga.  cirrus and qxl are left with a fixed
size (and private VGA_RAM_SIZE #define) for now.

qxl needs some non-trivial adjustments in the mode list handling deal
with a runtime-configurable size, which calls for a separate qxl patch.

cirrus emulates cards which have 2 MB (isa) and 4 MB (pci), so I guess
it would make sense to use these sizes.  That change would break
migration though, so I left it fixed at 8 MB size.  Making it
configurabls is pretty pointless for cirrus as we have to match real
hardware.

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