feat(network): make interfaces[] canonical, normalize flat fields as AWX compat

Flat network fields (bridge, ip, prefix, gateway, vlan) are now converted
into a single-entry interfaces[] list during normalization. All virtualization
tasks (proxmox, vmware, libvirt, xen) and configuration (NM, Alpine, Void)
now consume system_cfg.network.interfaces exclusively for multi-NIC support.

Also fixes: user.key -> user.keys in system_cfg output, strict list-only DNS
in example inventories, removes legacy single-MAC virtualization_mac_address
default.

templates/xen.cfg.j2

@@ -10,7 +10,11 @@ disk = [
   '{{ boot_iso }},,hdc,cdrom'{% if rhel_iso is defined and rhel_iso | length > 0 %}, '{{ rhel_iso }},,hdd,cdrom'{% endif %}
 {%- endif -%}
 ]
-vif = [ 'bridge={{ system_cfg.network.bridge }},model=e1000' ]
+vif = [
+{%- for iface in system_cfg.network.interfaces -%}
+  'bridge={{ iface.bridge }},model=e1000'{% if not loop.last %}, {% endif %}
+{%- endfor -%}
+]
 boot = "{{ 'dc' if xen_installer_media_enabled | bool else 'c' }}"
 on_crash = "preserve"
 on_poweroff = "destroy"