vm-virtio: checked_descriptor: Add unit test for boundary descriptor

Submit a descriptor whose buffer ends exactly at the last byte of
guest RAM and verify CheckedDescriptorIter accepts it. Guards
against an off by one in the range check that would reject an
otherwise valid descriptor at the memory boundary.

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

vm-virtio/src/checked_descriptor.rs

@@ -346,4 +346,20 @@ mod unit_tests {
         let result = it.next().expect("must yield an item");
         assert_eq!(result.unwrap_err(), GuestAddress(0x4000));
     }
+
+    #[test]
+    fn accepts_descriptor_at_memory_end() {
+        // 128 KiB of guest RAM ends at 0x20000. A descriptor whose buffer
+        // ends exactly at that boundary must be accepted.
+        const MEM_SIZE: usize = 128 * 1024;
+        const LEN: u32 = 256;
+        let addr = (MEM_SIZE as u64) - LEN as u64;
+        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 desc = it.next().unwrap().expect("boundary descriptor must be Ok");
+        assert_eq!(desc.addr().0, addr);
+        assert_eq!(desc.len(), LEN);
+    }
 }