Coverage: Fix #494, Ensure dma is not released.

Signed-off-by: xiaoyuxlu <[email protected]>

td-payload/src/bin/example/main.rs

@@ -77,6 +77,7 @@ pub extern "C" fn main() -> ! {
             buffer.as_ptr() as u64,
             coverage_len
         );
+        loop {}
     }
 
     panic!("td-payload: all tests finished and enters dead loop");