list: provide RPC call for snapshots

list: provide RPC call for snapshots

The generator doesn't handle lists of virDomainSnapshotPtr, so
this commit requires a bit more work than some RPC additions.

* src/remote/remote_protocol.x
(REMOTE_PROC_DOMAIN_LIST_ALL_SNAPSHOTS)
(REMOTE_PROC_DOMAIN_SNAPSHOT_LIST_ALL_CHILDREN): New RPC calls,
with corresponding structs.
* daemon/remote.c (remoteDispatchDomainListAllSnapshots)
(remoteDispatchDomainSnapshotListAllChildren): New functions.
* src/remote/remote_driver.c (remoteDomainListAllSnapshots)
(remoteDomainSnapshotListAllChildren): Likewise.
* src/remote_protocol-structs: Regenerate.