Fix shadowed variable warning

Fix shadowed variable warning

libvirtd.c: In function 'daemonSetupAccessManager':
libvirtd.c:730:18: error: declaration of 'driver' shadows
  a global declaration [-Werror=shadow]
     const char **driver = (const char **)config->access_drivers;
                  ^
In file included from libvirtd.c:95:0:
../src/node_device/node_device_driver.h:43:36: error: shadowed
  declaration is here [-Werror=shadow]
 extern virNodeDeviceDriverStatePtr driver;
                                    ^