Retyping an attached MA volume while attached to more than one instance should either be fixed blocked or documented as a limitation

If you retype a MA volume from one MA backend (LVM) to another MA backend(netapp) while the volume is attached only to a single instance it works fine. However if you attach the volume to a second instance this same migrations now fails, c-vol log reports a reason but if we know we can't make this work notify the user that this isn't supported.

We get an exception on c-vol log:
/novaclient/client.py", line 78, in request
2019-07-26 04:57:01.087 73 ERROR oslo_messaging.rpc.server raise exceptions.from_response(resp, body, url, method)
2019-07-26 04:57:01.087 73 ERROR oslo_messaging.rpc.server novaclient.exceptions.BadRequest: Swapping multi-attach volumes with more than one read/write attachment is not supported. (HTTP 400) (Request-ID: req-16c42279-9609-4fb1-a87e-db30687d7e11)
2019-07-26 04:57:01.087 73 ERROR oslo_messaging.rpc.server