Make `type sr` abstract?

In the IDL the `type sr` (for an attached SR) should probably be marked as abstract. We should return it from `SR.attach`, store it away somewhere and resupply it later.

For the python boilerplate this probably corresponds to using `pickle` to store away the result in the boilerplate. The `xapi-storage-service` could persist a mapping of `SR.uri` to `type sr` on disk.
