hw/ppc/spapr: Fix the selection of the processor features

The current code uses pa_features_206 for POWERPC_MMU_2_06, and
for everything else, it uses pa_features_207. This is bad in some
cases because there is also a "degraded" MMU version of ISA 2.06,
called POWERPC_MMU_2_06a, which should of course use the flags for
2.06 instead. And there is also the possibility that the user runs
the pseries machine with a POWER5+ or even 970 processor. In that
case we certainly do not want to set the flags for 2.07, and rather
simply skip the setting of the pa-features property instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit 4cbec30d)