vm-virtio: checked_descriptor: Add unit test for zero length descriptor

Add passes_through_zero_length_descriptor which submits a descriptor
with len 0 and verifies CheckedDescriptorIter yields it without
performing a guest memory range check.

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

vm-virtio/src/checked_descriptor.rs

@@ -238,4 +238,17 @@ mod unit_tests {
         let result = it.next().expect("iterator must yield an item");
         result.unwrap_err();
     }
+
+    #[test]
+    fn passes_through_zero_length_descriptor() {
+        let (_mem, mem_atomic, mut queue) = setup_vq(128 * 1024, 0x4000, 0, 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 desc = it
+            .next()
+            .unwrap()
+            .expect("zero length descriptor must pass through");
+        assert_eq!(desc.len(), 0);
+    }
 }