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.

roles/virtualization/templates/vm.xml.j2

@@ -46,11 +46,13 @@
       <target dev="sdc" bus="sata"/>
     </disk>
     {% endif %}
+    {% for iface in system_cfg.network.interfaces %}
     <interface type='network'>
-      <mac address="{{ virtualization_mac_address }}"/>
-      <source network='{{ system_cfg.network.bridge if (system_cfg.network.bridge | default('' ) | string | length) > 0 else "default" }}'/>
+      <mac address="{{ '52:54:00' | community.general.random_mac(seed=hostname if loop.index0 == 0 else hostname ~ '-nic' ~ loop.index0) }}"/>
+      <source network='{{ iface.bridge | default("default") }}'/>
       <model type='virtio'/>
     </interface>
+    {% endfor %}
     {% if virtualization_tpm2_enabled %}
     <tpm model='tpm-crb'>
       <backend type='emulator' version='2.0'/>