Rename services.virtualboxHost -> programs.virtualbox

VirtualBox is an application, not a system service.

nixos/modules/rename.nix

···
 # DNSCrypt-proxy
 ++ obsolete [ "services" "dnscrypt-proxy" "port" ] [ "services" "dnscrypt-proxy" "localPort" ]
 
+++ obsolete [ "services" "virtualboxHost" ] [ "programs" "virtualbox" ]
+
 # Options that are obsolete and have no replacement.
 ++ obsolete' [ "boot" "loader" "grub" "bootDevice" ]
 ++ obsolete' [ "boot" "initrd" "luks" "enable" ]

nixos/modules/virtualisation/virtualbox-host.nix

···
 with lib;
 
 let
-  cfg = config.services.virtualboxHost;
+  cfg = config.programs.virtualbox;
   virtualbox = config.boot.kernelPackages.virtualbox.override {
     inherit (cfg) enableHardening;
   };
···
 in
 
 {
-  options.services.virtualboxHost = {
+  options.programs.virtualbox = {
     enable = mkOption {
       type = types.bool;
       default = false;
       description = ''
-        Whether to enable host-side support for VirtualBox.
+        Whether to enable VirtualBox.
 
         <note><para>
           In order to pass USB devices from the host to the guests, the user