spapr: don't advertise radix GTSE if max-compat-cpu < power9

spapr: don't advertise radix GTSE if max-compat-cpu < power9

On a POWER9 host, if a guest runs in pre POWER9 compat mode, it necessarily
uses the hash MMU mode. In this case, we shouldn't advertise radix GTSE in
the ibm,arch-vec-5-platform-support DT property as the current code does.
The first reason is that it doesn't make sense, and the second one is that
causes the CAS-negotiated options subsection to be migrated. This breaks
backward migration to QEMU 2.7 and older versions on POWER8 hosts:

qemu-system-ppc64: error while loading state for instance 0x0 of device
 'spapr'
qemu-system-ppc64: load of migration failed: No such file or directory

This patch hence initialize CPUs a bit earlier so that we can check the
requested compat mode, and don't set OV5_MMU_RADIX_GTSE for power8 and
older.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>