htools: add more functions for local disk storage

These will be used in Node.hs for proper add/remove instance
code. Furthermore, we restrict the movable status to the right disk
templates only, so that we don't attempt to move the 'wrong' instance
types.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>