qemu-io: Clean up global variable shadowing

Fix:

  qemu-io.c:478:36: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
  static void add_user_command(char *optarg)
                                     ^
  /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/getopt.h:77:14: note: previous declaration is here
  extern char *optarg;                    /* getopt(3) external variables */
               ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231004120019.93101-8-philmd@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qemu-io.c

@@ -475,10 +475,10 @@ static int command_loop(void)
     return last_error;
 }
 
-static void add_user_command(char *optarg)
+static void add_user_command(char *user_cmd)
 {
     cmdline = g_renew(char *, cmdline, ++ncmdline);
-    cmdline[ncmdline-1] = optarg;
+    cmdline[ncmdline - 1] = user_cmd;
 }
 
 static void reenable_tty_echo(void)