api2/types/tape/drive: add changer_drivenum

so that an api user can see which drive belongs to which drivenum of a changer
for ones with multiple drives

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

src/api2/tape/changer.rs

@@ -166,6 +166,7 @@ pub fn list_changers(
             name: changer.name,
             path: changer.path.clone(),
             changer: None,
+            changer_drivenum: None,
             vendor: None,
             model: None,
             serial: None,