Detached replicas currently stay in the ACTIVE state
while detaching although the instance may not be available
throughout the whole process.
Set the task status to DETACHING until the guest finishes.
Update the scenario tests to expect this new instance state.
Also improve the assertion to provide more granular feedback.
Change-Id: Id43f382fa655bfa48feeb2521d7237d926883514
Closes-Bug: 1465600
Petr Malik
9e8020ef76