make datastore BackupGroup/Dir ctors private

And use the api-types for their contents.

These are supposed to be instances for a datastore, the pure
specifications are the ones in pbs_api_types which should be
preferred in crates like clients which do not need to deal
with the datastore directly.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

tests/prune.rs

@@ -29,7 +29,7 @@ fn get_prune_list(
 }
 
 fn create_info(snapshot: &str, partial: bool) -> BackupInfo {
-    let backup_dir: BackupDir = snapshot.parse().unwrap();
+    let backup_dir = BackupDir::new_test(snapshot.parse().unwrap());
 
     let mut files = Vec::new();