libxl: provide impl for nodeGetSecurityModel

Currently, the libxl driver does not support any security drivers.
When the qemu driver has no security driver configued,
nodeGetSecurityModel succeeds but returns an empty virSecurityModel
object.  Do the same in the libxl driver instead of reporting

this function is not supported by the connection driver:
virNodeGetSecurityModel

src/libxl/libxl_driver.c

@@ -5027,6 +5027,23 @@ libxlDomainMigrateConfirm3Params(virDomainPtr domain,
     return libxlDomainMigrationConfirm(driver, vm, flags, cancelled);
 }
 
+static int libxlNodeGetSecurityModel(virConnectPtr conn,
+                                     virSecurityModelPtr secmodel)
+{
+    memset(secmodel, 0, sizeof(*secmodel));
+
+    if (virNodeGetSecurityModelEnsureACL(conn) < 0)
+        return -1;
+
+    /*
+     * Currently the libxl driver does not support security model.
+     * Similar to the qemu driver, treat this as success and simply
+     * return no data in secmodel.  Avoids spamming the libvirt log
+     * with "this function is not supported by the connection driver:
+     * virNodeGetSecurityModel"
+     */
+    return 0;
+}
 
 static virHypervisorDriver libxlHypervisorDriver = {
     .name = LIBXL_DRIVER_NAME,
@@ -5122,6 +5139,7 @@ static virHypervisorDriver libxlHypervisorDriver = {
     .domainMigratePerform3Params = libxlDomainMigratePerform3Params, /* 1.2.6 */
     .domainMigrateFinish3Params = libxlDomainMigrateFinish3Params, /* 1.2.6 */
     .domainMigrateConfirm3Params = libxlDomainMigrateConfirm3Params, /* 1.2.6 */
+    .nodeGetSecurityModel = libxlNodeGetSecurityModel, /* 1.2.16 */
 };
 
 static virConnectDriver libxlConnectDriver = {