KVM: PPC: bookehv: add comment about shadow_msr

For BookE HV the guest visible MSR is shared->msr and is identical to
the MSR that is in use while the guest is running, because we can't trap
reads from/to MSR.

So shadow_msr is unused there. Indicate that with a comment.

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