tape: improve locking (lock media-sets)

- new helper: lock_media_set()

- MediaPool: lock media set

- Expose Inventory::new() to avoid double loading

- do not lock pool on restore (only lock media-set)

- change pool lock name to ".pool-{name}"

src/api2/tape/media.rs

@@ -122,7 +122,7 @@ pub async fn list_media(
         let config: MediaPoolConfig = config.lookup("pool", pool_name)?;
 
         let changer_name = None; // assume standalone drive
-        let mut pool = MediaPool::with_config(status_path, &config, changer_name)?;
+        let mut pool = MediaPool::with_config(status_path, &config, changer_name, true)?;
 
         let current_time = proxmox::tools::time::epoch_i64();