vm-virtio: checked_descriptor: Add unit test for GVA translation

Introduce an OffsetTranslator stub implementing AccessPlatform and
verify CheckedDescriptorIter applies the translation, so the
yielded descriptor's addr reflects the translated GPA rather than
the raw descriptor address.

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

vm-virtio/src/checked_descriptor.rs

@@ -392,4 +392,42 @@ mod unit_tests {
         let result = it.next().expect("must yield an item");
         assert_eq!(result.unwrap_err(), GuestAddress(addr));
     }
+
+    /// AccessPlatform stub that applies a fixed offset between GVA and GPA.
+    #[derive(Debug)]
+    struct OffsetTranslator {
+        gva_base: u64,
+        gpa_base: u64,
+    }
+
+    impl AccessPlatform for OffsetTranslator {
+        fn translate_gva(&self, base: u64, _size: u64) -> std::io::Result<u64> {
+            Ok(base - self.gva_base + self.gpa_base)
+        }
+        fn translate_gpa(&self, base: u64, _size: u64) -> std::io::Result<u64> {
+            Ok(base - self.gpa_base + self.gva_base)
+        }
+    }
+
+    #[test]
+    fn translates_descriptor_via_access_platform() {
+        const MEM_SIZE: usize = 128 * 1024;
+        const LEN: u32 = 256;
+        let gva = 0x1_0000_0000u64;
+        let gpa = 0x4000u64;
+        let (_mem, mem_atomic, mut queue) = setup_vq(MEM_SIZE, gva, LEN, 0);
+        let mem_guard = mem_atomic.memory();
+        let mut chain = queue.pop_descriptor_chain(mem_guard).unwrap();
+        let ap = OffsetTranslator {
+            gva_base: gva,
+            gpa_base: gpa,
+        };
+        let mut it = chain.checked_iter(Some(&ap));
+        let desc = it
+            .next()
+            .unwrap()
+            .expect("translated descriptor must be Ok");
+        assert_eq!(desc.addr().0, gpa);
+        assert_eq!(desc.len(), LEN);
+    }
 }