xenapi: Fix build after const correctnes changes

xenapi: Fix build after const correctnes changes

In commit d24677090f1c0596ac1585cc233d6e130b9bb75f the header of one of
the post parse callbacks was changed, but the function used as the
callback in the xenapi driver wasn't adapted. This resulted into:

  CC       xenapi/libvirt_driver_xenapi_la-xenapi_utils.lo
 xenapi/xenapi_driver.c:63:5: error: initialization from incompatible pointer type [-Werror]
 xenapi/xenapi_driver.c:63:5: error: (near initialization for 'xenapiDomainDefParserConfig.devicesPostParseCallback') [-Werror]