add nicModel option, restore MAS activation with LTSC key fix

- Add nicModel option (default: virtio-net-pci) to allow e1000 for
  images without VirtIO drivers
- Restore MAS activation with slmgr /ipk to switch back from IoT
  Enterprise S to Enterprise LTSC (which has native RDP server)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nixos/vms/config.nix

@@ -208,14 +208,14 @@ let
           '') vmCfg.shares)} \
           ${optionalString cfg.networks.user.enable "
             -netdev user,id=user \
-            -device virtio-net-pci,netdev=user \
+            -device ${vmCfg.nicModel},netdev=user \
           "} \
           ${concatMapStrings (tapCfg: ''
-            -device virtio-net-pci,netdev=lan-${tapCfg.name},mac=${tapCfg.mac} \
+            -device ${vmCfg.nicModel},netdev=lan-${tapCfg.name},mac=${tapCfg.mac} \
             -netdev tap,id=lan-${tapCfg.name},ifname=${tapCfg.iface},script=no,downscript=no \
           '') allTaps} \
           ${concatStrings (imap1 (i: macvtap: ''
-            -device virtio-net-pci,netdev=macvtap-${macvtap.name},mac=$(ip l show ${macvtap.iface} | awk '/link\/ether/{print $2}') \
+            -device ${vmCfg.nicModel},netdev=macvtap-${macvtap.name},mac=$(ip l show ${macvtap.iface} | awk '/link\/ether/{print $2}') \
             -netdev tap,id=macvtap-${macvtap.name},fd=${toString (i+2)} ${toString (i+2)}<>/dev/tap$(ip l show ${macvtap.iface} | awk -F':' '/${macvtap.iface}/{print $1}') \
           '') allMacvtaps)} \
           ${concatStrings (imap1 (i: pciAddr: ''

nixos/vms/submoduleOptions.nix

@@ -235,6 +235,12 @@ with lib;
       description = "Enable boot menu.";
     };
 
+    nicModel = mkOption {
+      type = types.str;
+      default = "virtio-net-pci";
+      description = "QEMU NIC device model (e.g. virtio-net-pci, e1000).";
+    };
+
     windows = {
       enable = mkOption {
         type = types.bool;