build: fix missing initializer

Commit cb4aba9b forgot xenapi.

* src/xenapi/xenapi_driver.c (xenapiDriver): Adjust to recent API.

src/xenapi/xenapi_driver.c

@@ -1868,6 +1868,7 @@ static virDriver xenapiDriver = {
     NULL, /* domainGetJobInfo */
     NULL, /* domainAbortJob */
     NULL, /* domainMigrateSetMaxDowntime */
+    NULL, /* domainMigrateSetMaxSpeed */
     NULL, /* domainEventRegisterAny */
     NULL, /* domainEventDeregisterAny */
     NULL, /* domainManagedSave */