KVM: SEV: do not use list_replace_init on an empty list

list_replace_init cannot be used if the source is an empty list,
because "new->next->prev = new" will overwrite "old->next":

				new				old
				prev = new, next = new		prev = old, next = old
new->next = old->next		prev = new, next = old		prev = old, next = old
new->next->prev = new		prev = new, next = old		prev = old, next = new
new->prev = old->prev		prev = old, next = old		prev = old, next = old
new->next->prev = new		prev = old, next = old		prev = new, next = new

The desired outcome instead would be to leave both old and new the same
as they were (two empty circular lists).  Use list_cut_before, which
already has the necessary check and is documented to discard the
previous contents of the list that will hold the result.

Fixes: b56639318b ("KVM: SEV: Add support for SEV intra host migration")
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211123005036.2954379-5-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/sev.c

@@ -1613,8 +1613,7 @@ static void sev_migrate_from(struct kvm_sev_info *dst,
 	src->handle = 0;
 	src->pages_locked = 0;
 
-	INIT_LIST_HEAD(&dst->regions_list);
+	list_cut_before(&dst->regions_list, &src->regions_list, &src->regions_list);
-	list_replace_init(&src->regions_list, &dst->regions_list);
 }
 
 static int sev_es_migrate_from(struct kvm *dst, struct kvm *src)