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>

arch/powerpc/include/asm/kvm_host.h

@@ -386,6 +386,7 @@ struct kvm_vcpu_arch {
 #endif
 	u32 vrsave; /* also USPRG0 */
 	u32 mmucr;
+	/* shadow_msr is unused for BookE HV */
 	ulong shadow_msr;
 	ulong csrr0;
 	ulong csrr1;