Disable volume attach/detach for suspended instances

As described in the bug - some hypervisors (libvirt) do not support
this. It is best to disable it in the API to provide a consistent user
experience.

Also adds a test to prevent an accidental regression.

Change-Id: I5b404cca22cffecbaf524e2511810e5341242052
Closes-bug: #1240922
Merge "Disable volume attach/detach for suspended instances"