Fixes migrate_volume_completion

The following issues are addressed:

The source volume is never deleted when a migration is done because
status_update was assumed to always be a dict.

Unattached volumes are set to in-use when retyped.

In-use migrated volumes are left in status attaching.

Change-Id: Ib5034679a5f469ba53e6280a47e8c94d633cf911
Partial-Bug: 1316079
Merge "Fixes migrate_volume_completion"