The bochs-display mmio bar has some sub-regions with the actual hardware
registers.  What happens when the guest access something outside those
regions depends on the archirecture.  On x86 those reads succeed (and
return 0xff I think).  On risc-v qemu aborts.

This patch adds handlers for the parent region, to make the wanted
behavior explicit and to make things consistent across architectures.

v2:
 - use existing unassigned_io_ops.
 - also cover stdvga.

Cc: Alistair Francis <alistair23@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20200309100009.17624-1-kraxel@redhat.com