Pass PVR in sregs

Right now sregs is unused on PPC, so we can use it for initialization
of the CPU.

KVM on BookE always virtualizes the host CPU. On Book3s we go a step further
and take the PVR from userspace that tells us what kind of CPU we are supposed
to virtualize, because we support Book3s_32 and Book3s_64 guests.

In order to get that information, we use the sregs ioctl, because we don't
want to reset the guest CPU on every normal register set.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>