KVM: powerpc: Fix mtsrin in book3s_64 mmu

We were shifting the Ks/Kp/N bits one bit too far on mtsrin. It took
me some time to figure that out, so I also put in some debugging and a
comment explaining the conversion.

This fixes current OpenBIOS boot on PPC64 KVM.

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