vga: Add endian to vmstate

Include the endian state in the migration stream as an optional
subsection which we only include when the endian isn't the default,
thus enabling backward compatibility of the common case.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Changes by kraxel:
 * Remove bochs dispi interface changes.  We'll do that in
   a different way to make sure we don't conflict with
   possible future bochs dispi interface changes.
 * keep live migration bits.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>