tape: MediaPool - replace use_offline_media with changer_name

This way, we can improve location_is_available, because we only
consider media from that changer as available.

src/api2/tape/media.rs

@@ -86,8 +86,8 @@ pub async fn list_media(pool: Option<String>) -> Result<Vec<MediaListEntry>, Err
 
         let config: MediaPoolConfig = config.lookup("pool", pool_name)?;
 
-        let use_offline_media = true; // does not matter here
-        let pool = MediaPool::with_config(status_path, &config, use_offline_media)?;
+        let changer_name = None; // does not matter here
+        let pool = MediaPool::with_config(status_path, &config, changer_name)?;
 
         let current_time = proxmox::tools::time::epoch_i64();