Add G_GNUC_PRINTF to function qemu_set_info_str and fix related issues

With the G_GNUC_PRINTF function attribute the compiler detects
two potential insecure format strings:

../../../net/stream.c:248:31: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
    qemu_set_info_str(&s->nc, uri);
                              ^~~
../../../net/stream.c:322:31: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
    qemu_set_info_str(&s->nc, uri);
                              ^~~

There are also two other warnings:

../../../net/socket.c:182:35: warning: zero-length gnu_printf format string [-Wformat-zero-length]
  182 |         qemu_set_info_str(&s->nc, "");
      |                                   ^~
../../../net/stream.c:170:35: warning: zero-length gnu_printf format string [-Wformat-zero-length]
  170 |         qemu_set_info_str(&s->nc, "");

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221126152507.283271-7-sw@weilnetz.de>