tape: fix snapshot path in catalog and snapshot_archive

tape: fix snapshot path in catalog and snapshot_archive

both used the 'Display' trait of pbs_datastore::BackupDir, which is not
intended to be serialized anywhere. Instead, manually format the path
using the print_ns_and_snapshot helper, and conversely, parse with
'parse_ns_and_snapshot'. to be a bit safer, change the register_snapshot
signature to take a BackupNamespace and BackupDir instead of a string.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>