open images readonly

we don't support any write commands anyways

src/main.rs

@@ -82,7 +82,6 @@ async fn run() -> Result<()> {
         let mut dev = BlockDevice::new(FileBackend::new(
             File::options()
                 .read(true)
-                .write(true)
                 .open(image)
                 .expect("Opening image"),
         ));