Ensure failure to talk to Xen hypervisor is fatal when privileged

As per the comment, the Xen hypervisor driver is considered to
be mandatory when running privileged. When it fails to open,
we should thus return an error, not ignore it.
(cherry picked from commit 489900e3)

src/xen/xen_driver.c

@@ -340,6 +340,8 @@ xenUnifiedOpen (virConnectPtr conn, virConnectAuthPtr auth, unsigned int flags)
         if (xenHypervisorOpen(conn, auth, flags) == VIR_DRV_OPEN_SUCCESS) {
             VIR_DEBUG("Activated hypervisor sub-driver");
             priv->opened[XEN_UNIFIED_HYPERVISOR_OFFSET] = 1;
+        } else {
+            goto fail;
         }
     }