tree-wide: prefer api-type BackupGroup for logging

together with DatastoreWithNamespace where needed, to not forget
namespace information.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

pbs-datastore/examples/ls-snapshots.rs

@@ -25,7 +25,7 @@ fn run() -> Result<(), Error> {
 
         for group in store.iter_backup_groups(ns)? {
             let group = group?;
-            println!("    found group {}", group);
+            println!("    found group {}", group.group());
 
             for snapshot in group.iter_snapshots()? {
                 let snapshot = snapshot?;