Rewrite init script in rust and fix non-shell CMDs

src/qemu.rs

@@ -85,7 +85,9 @@ pub(crate) fn run(
     }
 
     if let Some(cmd) = &cmd {
-        let encoded = base64::engine::general_purpose::STANDARD.encode(cmd.as_bytes());
+        let argv = ["/bin/sh".to_string(), "-c".to_string(), cmd.clone()];
+        let data: Vec<u8> = argv.join("\0").into_bytes();
+        let encoded = base64::engine::general_purpose::STANDARD.encode(&data);
         cmdline.push(format!("slim.cmd={encoded}"));
     }