ppc/pnv: introduce a new isa_create() operation to the chip model

ppc/pnv: introduce a new isa_create() operation to the chip model

This moves the details of the ISA bus creation under the LPC model but
more important, the new PnvChip operation will let us choose the chip
class to use when we introduce the different chip classes for Power9
and Power8. It hides away the processor chip controllers from the
machine.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>