hw/nxagent/Rootless.c: Type cast type Atom to type long unsigned int before handing it over to %lu in fprintf().

 Amends the following compiler warnings:

 ```
 Rootless.c: In function ‘nxagentExportProperty’:
 Rootless.c:775:21: warning: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 3 has type ‘Atom’ [-Wformat=]
                      "Property [%lu] too long.\n", propertyX);
                      ^
 ```