hw/core/register: Mark the device with cannot_instantiate_with_device_add_yet

hw/core/register: Mark the device with cannot_instantiate_with_device_add_yet

The "qemu,register" device needs to be wired up in source code, there
is no way the user can make any real use of this device with the
"-device" parameter or the "device_add" monitor command yet.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>