Remove the jointly loaded model in finish_volume_migration

Remove the jointly loaded model in finish_volume_migration

After a successful volume migration, the source volume and the
destination volume need to swap the data in the models in
finish_volume_migration. It is sufficient to load the volume
model only and there is no need to load other models, like
volume type, metadata, consistency group, etc. If we load
these additional models, it will lead to NULL key error, when
either source or destination model has a NULL key pointer.

Change-Id: I04ad0739387d602719591680854e6655cc87f9ab
Closes-Bug: #1505572