ppc/pnv: change the PowerNV machine devices to be non user creatable

ppc/pnv: change the PowerNV machine devices to be non user creatable

The PowerNV machine emulates an OpenPOWER system and the PowerNV chip
devices are models of the internal logic of the POWER processor. They
can not be instantiated by the user on the QEMU command line.

The PHB3/PHB4 devices could be an exception in the future after some
rework on how the device tree is built. For the moment, exclude them
also.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20200129113720.7404-1-clg@kaod.org>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>