reference the datastore in BackupGroup/Dir

And drop the base_path parameter on a first bunch of
functions (more reordering will follow).

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

src/api2/tape/backup.rs

@@ -458,7 +458,7 @@ fn backup_worker(
         progress.done_snapshots = 0;
         progress.group_snapshots = 0;
 
-        let snapshot_list = group.list_backups(&datastore.base_path())?;
+        let snapshot_list = group.list_backups()?;
 
         // filter out unfinished backups
         let mut snapshot_list: Vec<_> = snapshot_list