vm-virtio: checked_descriptor: Add unit test for exhausted chain

After the iterator yields the only descriptor in a chain, the
subsequent None must reflect exhaustion rather than a validation
failure, so failed and failed_addr stay unset.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

vm-virtio/src/checked_descriptor.rs

@@ -458,4 +458,16 @@ mod unit_tests {
         // translated one.
         assert_eq!(result.unwrap_err(), GuestAddress(gva));
     }
+
+    #[test]
+    fn exhausted_chain_returns_none() {
+        // After the iterator yields the only descriptor in the chain, the
+        // subsequent None must reflect exhaustion, not a validation failure.
+        let (_mem, mem_atomic, mut queue) = setup_vq(128 * 1024, 0x4000, 256, 0);
+        let mem_guard = mem_atomic.memory();
+        let mut chain = queue.pop_descriptor_chain(mem_guard).unwrap();
+        let mut it = chain.checked_iter(None);
+        it.next().unwrap().unwrap();
+        assert!(it.next().is_none());
+    }
 }