Mostly straight-forward, although this is the first API that
returns a new snapshot based on a snapshot rather than a domain.

* src/remote/remote_protocol.x
(REMOTE_PROC_DOMAIN_SNAPSHOT_GET_PARENT): New rpc.
(remote_domain_snapshot_get_parent_args)
(remote_domain_snapshot_get_parent_ret): New structs.
* src/rpc/gendispatch.pl: Adjust generator.
* src/remote/remote_driver.c (remote_driver): Use it.
* src/remote_protocol-structs: Update.