Fix format of MAAS curtin:xenial:amd64

Change-Id: I9884aa33508ea5097f88e0e138f7577166c9a57d
Related-bug: PROD-25243 (PROD:25243)
(cherry picked from commit e5a5bb51287bad1e6b3724d2c59eb9ecd90c53e8)
diff --git a/maas/region/single.yml b/maas/region/single.yml
index 9d60882..a435309 100644
--- a/maas/region/single.yml
+++ b/maas/region/single.yml
@@ -14,8 +14,12 @@
       curtin_vars:
         amd64:
           xenial:
-            extra_pkgs: [ "linux-headers-virtual-hwe-16.04", "linux-image-extra-virtual-hwe-16.04" ]
-            kernel_package: 'linux-image-virtual-hwe-16.04'
+            extra_pkgs:
+              enabled: true
+              pkgs: [ "linux-headers-virtual-hwe-16.04", "linux-image-extra-virtual-hwe-16.04" ]
+            kernel_package:
+              enabled: true
+              value: 'linux-image-virtual-hwe-16.04'
     region:
       salt_master_ip: ${_param:infra_config_deploy_address}
       theme: mirantis