vzdump: backup limit: only count unprotected backups

vzdump: backup limit: only count unprotected backups

since they are the ones relevant for pruning and protected backups
have their own separate limit.

Since get_backup_file_list is only used in places where the
unprotected backups are needed, adapt the helper accordingly.

If there is a storage, use PVE::Storage::volume_list to count the
unprotected backups. This avoids a direct invocation of the
proxmox-backup-client for PBS and the limit check can also work for
external storage plugins which might not be dir-based or name the
backups differently.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>