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
Add note that some checks are disabled on purpose

Change-Id: I9a2daf57a8bb73e292d99f5177c310d75061cc13
Merge "Fixes migrate_volume_completion"
Merge "Add note that some checks are disabled on purpose"