Prevent volume-attach/detach from instances in rescue state

Rescue is supposed to just be a way to log into a wayward instance
if something goes wrong with the base image that may have had some data
(logfiles etc.) and make it possible to grab that  - block devices are
assumed to be accessible by re-attaching them, and are considered
persistant so no need for rescue there.

Since there is no need to attach a volume in rescue mode, and detaching a
volume that was previously attached doesn't work, just ban volume
attach/detach in rescue state.

Fixes bug 1126187

Change-Id: Ifdf164155942cdeb2bbdfcd1dce0dd2e125b507c
Merge "Prevent volume-attach/detach from instances in rescue state"