ui: tape: handle tapes in changers without barcode

by checking for definedness of the label (tapes without barcode
have the empty string as label-text) and falling back to the
source slot for the load action

Note: Changed the load-slot API from PUT to POST

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

src/api2/tape/drive.rs

@@ -1453,7 +1453,7 @@ pub const SUBDIRS: SubdirMap = &sorted!([
     (
         "load-slot",
         &Router::new()
-            .put(&API_METHOD_LOAD_SLOT)
+            .post(&API_METHOD_LOAD_SLOT)
     ),
     (
         "cartridge-memory",