ppc/spapr: fix default cpu for pre-9.0 machines.

ppc/spapr: fix default cpu for pre-9.0 machines.

When POWER10 CPU was made as default, we missed keeping POWER9 as
default for older pseries releases (pre-9.0) at that time.
This caused breakge in default cpu evaluation for older pseries
machines and hence this fix.

Fixes: 51113013f3 ("ppc/spapr: change pseries machine default to POWER10 CPU")
Cc: qemu-stable@nongnu.org
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250313094705.2361997-1-harshpb@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
(cherry picked from commit 1490d0bcdfcb78b4503cae42353d3dd50f4e9d96)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>