backup: ensure base snapshots are still available after backup

This should never trigger if everything else works correctly, but it is
still a very cheap check to avoid wrongly marking a backup as "OK" when
in fact some chunks might be missing.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/backup/backup_info.rs

@@ -217,7 +217,7 @@ impl std::str::FromStr for BackupGroup {
 /// Uniquely identify a Backup (relative to data store)
 ///
 /// We also call this a backup snaphost.
-#[derive(Debug, Eq, PartialEq, Clone)]
+#[derive(Debug, Eq, PartialEq, Hash, Clone)]
 pub struct BackupDir {
     /// Backup group
     group: BackupGroup,