snapshot: RPC for new query APIs

snapshot: RPC for new query APIs

Pretty straightforward.

* src/remote/remote_protocol.x
(remote_domain_snapshot_is_current_args)
(remote_domain_snapshot_is_current_ret)
(remote_domain_snapshot_has_metadata_args)
(remote_domain_snapshot_has_metadata_ret): New structs.
(REMOTE_PROC_DOMAIN_SNAPSHOT_IS_CURRENT)
(REMOTE_PROC_DOMAIN_SNAPSHOT_HAS_METADATA): New RPC calls.
* src/remote/remote_driver.c (remote_driver): Call them.
* src/remote_protocol-structs: Regenerate.