KVM: const-ify uses of struct kvm_userspace_memory_region

Architecture-specific helpers are not supposed to muck with
struct kvm_userspace_memory_region contents.  Add const to
enforce this.

In order to eliminate the only write in __kvm_set_memory_region,
the cleaning of deleted slots is pulled up from update_memslots
to __kvm_set_memory_region.

Reviewed-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Reviewed-by: Radim Krcmar <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/powerpc/kvm/book3s_pr.c

@@ -1573,13 +1573,13 @@ static void kvmppc_core_flush_memslot_pr(struct kvm *kvm,
 
 static int kvmppc_core_prepare_memory_region_pr(struct kvm *kvm,
 					struct kvm_memory_slot *memslot,
-					struct kvm_userspace_memory_region *mem)
+					const struct kvm_userspace_memory_region *mem)
 {
 	return 0;
 }
 
 static void kvmppc_core_commit_memory_region_pr(struct kvm *kvm,
-				struct kvm_userspace_memory_region *mem,
+				const struct kvm_userspace_memory_region *mem,
 				const struct kvm_memory_slot *old)
 {
 	return;