Use object for blockdev_find RPC call result

This patch changes the return type for backend.BlockdevFind to an object
(objects.BlockDevStatus). Before a tuple was used. Adding more values to
this tuple causes a lot of work.  Converting the result to an object with
properties will make this a bit simpler.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>