xenapi: Fix build after const correctnes changes

In commit d2467709 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]

src/xenapi/xenapi_driver.c

@@ -45,7 +45,7 @@
 
 static int
 xenapiDomainDeviceDefPostParse(virDomainDeviceDefPtr dev,
-                               virDomainDefPtr def,
+                               const virDomainDef *def,
                                virCapsPtr caps ATTRIBUTE_UNUSED,
                                void *opaque ATTRIBUTE_UNUSED)
 {