vga: Expose framebuffer byteorder as a QOM property

The VGA device model now supports having the framebuffer in either endian,
and can be switched between these by the guest via a register in the qext
region.

However, in some cases (e.g. LE OS on the pseries machine) we have
existing guest that don't know about the endian switch register, but other
parts of the qemu code have better information to set a default endianness
than the VGA code does of itself.

In order to allow them to set a correct default endianness in these cases,
without breaking abstraction walls, this patch exposes the VGA framebuffer
endianness via a writable QOM property.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
[agraf: use instance_init for property exposure]
Signed-off-by: Alexander Graf <agraf@suse.de>
