hw/ppc/spapr: Use POWER8 by default for the pseries-2.8 machine

A couple of distributors are compiling their distributions
with "-mcpu=power8" for ppc64le these days, so the user sooner
or later runs into a crash there when not explicitely specifying
the "-cpu POWER8" option to QEMU (which is currently using POWER7
for the "pseries" machine by default). Due to this reason, the
linux-user target already switched to POWER8 a while ago (see commit
de3f1b98410e0d5b406a0df3a48547b559d18602). Since the softmmu target
of course has the same problem, we should switch there to POWER8 for
the newer machine types, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
