refactor(global_defaults): data-driven hypervisor validation and shared constants

roles/virtualization/tasks/libvirt.yml

@@ -3,7 +3,7 @@
   ansible.builtin.set_fact:
     virtualization_libvirt_disks: "{{ virtualization_libvirt_disks | default([]) + [virtualization_libvirt_disk_cfg] }}"
   vars:
-    device_letter_map: "abcdefghijklmnopqrstuvwxyz"
+    device_letter_map: "{{ disk_letter_map }}"
     device_letter: "{{ device_letter_map[ansible_loop.index0] }}"
     virtualization_libvirt_disk_cfg: >-
       {{

roles/virtualization/tasks/xen.yml

@@ -5,7 +5,7 @@
       ansible.builtin.set_fact:
         virtualization_xen_disks: "{{ virtualization_xen_disks | default([]) + [virtualization_xen_disk_cfg] }}"
       vars:
-        device_letter_map: "abcdefghijklmnopqrstuvwxyz"
+        device_letter_map: "{{ disk_letter_map }}"
         device_letter: "{{ device_letter_map[ansible_loop.index0] }}"
         virtualization_xen_disk_cfg: >-
           {{