Convert export_info rpc to new style result

This also removes some code from ganeti-noded and rpc.py, which should
not do such processing of data (and be simply glue code). (Or
alternatively they could, if we had better infrastructure).

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