virtio-devices: balloon: Report used length of 0 for inflate/deflate

The inflate and deflate queues carry device-readable PFN arrays: the
driver writes them and the device only reads. The device never writes
to the descriptor buffers, so the used ring entry should report 0
bytes written rather than the descriptor length.

Signed-off-by: Rob Bradford <rbradford@meta.com>
Assisted-by: Claude:claude-opus-4-6

virtio-devices/src/balloon.rs

@@ -322,7 +322,7 @@ impl BalloonEpollHandler {
             }
 
             self.queues[queue_index]
-                .add_used(desc_chain.memory(), desc_chain.head_index(), desc.len())
+                .add_used(desc_chain.memory(), desc_chain.head_index(), 0)
                 .map_err(Error::QueueAddUsed)?;
             used_descs = true;
         }