Implement forgotten backend of virInterfaceIsActive()

Somehow the backend of this function was never implemented in
libvirt's netcf driver, and nobody noticed until now. (The required
netcf function was already in place, so nothing needs to change
there.)
* src/interface/netcf_driver.c: add in the backend function, and point
                                to it from the table of driver functions.

src/interface/netcf_driver.c

@@ -511,6 +511,38 @@ cleanup:
     return ret;
 }
 
+static int interfaceIsActive(virInterfacePtr ifinfo)
+{
+    struct interface_driver *driver = ifinfo->conn->interfacePrivateData;
+    struct netcf_if *iface = NULL;
+    unsigned int flags = 0;
+    int ret = -1;
+
+    interfaceDriverLock(driver);
+
+    iface = interfaceDriverGetNetcfIF(driver->netcf, ifinfo);
+    if (!iface) {
+        /* helper already reported error */
+        goto cleanup;
+    }
+
+    if (ncf_if_status(iface, &flags) < 0) {
+        const char *errmsg, *details;
+        int errcode = ncf_error(driver->netcf, &errmsg, &details);
+        interfaceReportError(netcf_to_vir_err(errcode),
+                             "failed to get status of interface %s (netcf: %s - %s)",
+                             ifinfo->name, errmsg, details ? details : "");
+        goto cleanup;
+    }
+
+    ret = flags & NETCF_IFACE_ACTIVE ? 1 : 0;
+
+cleanup:
+    ncf_if_free(iface);
+    interfaceDriverUnlock(driver);
+    return ret;
+}
+
 static virInterfaceDriver interfaceDriver = {
     "Interface",
     interfaceOpenInterface,          /* open */
@@ -526,7 +558,7 @@ static virInterfaceDriver interfaceDriver = {
     interfaceUndefine,               /* interfaceUndefine */
     interfaceCreate,                 /* interfaceCreate */
     interfaceDestroy,                /* interfaceDestroy */
-    NULL,                            /* interfaceIsActive */
+    interfaceIsActive,               /* interfaceIsActive */
 };
 
 int interfaceRegister(void) {