KVM: PPC: Set VSID_PR also for Book3S_64

Book3S_64 didn't set VSID_PR when we're in PR=1. This lead to pretty bad
behavior when searching for the shadow segment, as part of the code relied
on VSID_PR being set.

This patch fixes booting Book3S_64 guests.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>