NSX: Fix fake_api_client to raise NotFound

If one quries NSX doing GET /ws.v1/lswitch/LS_UUID/lport and LS_UUID
is a UUID that does not exist in NSX. NSX raises a 404. If LS_UUID is *
NSX returns an empty result string. This patch fixes the fake_api_client
so that it's behavior is correct.

Change-Id: Id66299d6ae3cfa43a65d4cb28f34348d64d8ed65
Closes-bug: 1311291
(cherry picked from commit dd143a619900792a0563b9ac4fe1f78197291d94)
Merge "NSX: Fix fake_api_client to raise NotFound" into stable/icehouse