KVM: PPC: Add support for explicit HIOR setting

Until now, we always set HIOR based on the PVR, but this is just wrong.
Instead, we should be setting HIOR explicitly, so user space can decide
what the initial HIOR value is - just like on real hardware.

We keep the old PVR based way around for backwards compatibility, but
once user space uses the SREGS based method, we drop the PVR logic.

Signed-off-by: Alexander Graf <agraf@suse.de>