mge-xml: fix compile-time warnings, versioning

```
../../drivers/mge-xml.c: In function ‘vvalue_mge_xml2nut’:
../../drivers/mge-xml.c:1492:20: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
      char *vconv = info->convert(vcpy);
                    ^
../../drivers/mge-xml.c: In function ‘vname_register_rw’:
../../drivers/mge-xml.c:1513:4: warning: zero-length gnu_printf format string [-Wformat-zero-length]
    dstate_setinfo(info->nutname, "");
    ^
```

..as we are now taking care of these things (https://github.com/networkupstools/nut/commit/2cc43bda48e27a81782db6a848389bc722790053 & https://github.com/networkupstools/nut/commit/7daa0feb6ed4f1c29bfe14c8e491ba198a4ba643)