vm-virtio: checked_descriptor: Add unit test for failed_addr

Drive CheckedDescriptorIter with an out of range descriptor and
assert that failed_addr returns Some carrying the GuestAddress of
the rejected descriptor, not just the failed boolean.

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

vm-virtio/src/checked_descriptor.rs

@@ -336,4 +336,14 @@ mod unit_tests {
             .expect_err("validation must reject the descriptor");
         assert_eq!(err.0, 0x4000);
     }
+
+    #[test]
+    fn err_reports_rejected_address() {
+        let (_mem, mem_atomic, mut queue) = setup_vq(128 * 1024, 0x4000, 1 << 30, 0);
+        let mem_guard = mem_atomic.memory();
+        let mut chain = queue.pop_descriptor_chain(mem_guard).unwrap();
+        let mut it = chain.checked_iter(None);
+        let result = it.next().expect("must yield an item");
+        assert_eq!(result.unwrap_err(), GuestAddress(0x4000));
+    }
 }