Make the current "powernv" machine an abstract type and derive from it
new machines with specific CPU models: power8 and power9.

The "powernv" machine is now an alias on the "powernv9" machine.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190731141233.1340-2-clg@kaod.org>
[dwg: Adjust pnv-xscom-test to cope with this change]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>