Allow Volume Operations When Shelved

This spec-less blueprint enables the attach volume and detach volume operations when an instance is in shelved or shelved_offloaded states.

At the moment volumes cannot be attached or detached when an instance has been shelved. These operations can be done when the instance is paused or stopped so it seems odd not to do it for shelved instances as well.

The case for the shelved state is similar to stopped or paused because the instance is still on the host, so this is covered by changing the allowed states for detach_volume() and attach_volume() in the compute API.

The case for the shelved_offloaded state requires a couple of new methods for attach and detach similar to the way bdms are dealt with in _local_delete() in the compute API class which deletes instances that have been offloaded.