spapr: fallback to raw mode if best compat mode cannot be set during CAS

KVM PR doesn't allow to set a compat mode. This causes ppc_set_compat_all()
to fail and we return H_HARDWARE to the guest right away.

This is excessive: even if we favor compat mode since commit 152ef803,
we should at least fallback to raw mode if the guest supports it.

This patch modifies cas_check_pvr() so that it also reports that the real
PVR was found in the table supplied by the guest. Note that this is only
makes sense if raw mode isn't explicitely disabled (ie, the user didn't
set the machine "max-cpu-compat" property). If this is the case, we can
simply ignore ppc_set_compat_all() failures, and let the guest run in raw
mode.

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