fix(runtime): migrate roles to nested system fields

templates/xen.cfg.j2

@@ -1,6 +1,6 @@
 builder = "hvm"
 name = "{{ hostname }}"
-memory = "{{ system_cfg.memory_mb }}"
+memory = "{{ system_cfg.memory }}"
 vcpus = "{{ system_cfg.cpus }}"
 disk = [
 {%- for disk in virtualization_xen_disks | default([]) -%}