ConfigWriter.RenameInstance: Stop using iv_name, safer operation

Stop using the disk index encoded in “iv_name” when renaming an instance.

This patch also changes the code to operate on a copy of the instance
until the major changes have been applied. In the case of a failure we
won't loose the instance object anymore.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>