vm-virtio: checked_descriptor: Add unit test for one past memory end

Submit a descriptor whose buffer extends exactly one byte past the
end of guest RAM and verify CheckedDescriptorIter rejects it, with
failed_addr returning the descriptor's start address.

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

vm-virtio/src/checked_descriptor.rs

@@ -362,4 +362,19 @@ mod unit_tests {
         assert_eq!(desc.addr().0, addr);
         assert_eq!(desc.len(), LEN);
     }
+
+    #[test]
+    fn rejects_descriptor_one_past_memory_end() {
+        // A descriptor whose buffer extends one byte past the end of guest
+        // RAM must be rejected.
+        const MEM_SIZE: usize = 128 * 1024;
+        const LEN: u32 = 256;
+        let addr = (MEM_SIZE as u64) - LEN as u64 + 1;
+        let (_mem, mem_atomic, mut queue) = setup_vq(MEM_SIZE, addr, LEN, 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(addr));
+    }
 }