nixos/vbox: Move all options to virtualisation.*.

Commit 687caeb renamed services.virtualboxHost to programs.virtualbox,
but according to the discussion on the commit, it's probably a better to
put it into virtualisation.virtualbox instead.

The discussion can be found here:

https://github.com/NixOS/nixpkgs/commit/687caeb#commitcomment-12664978

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/doc/manual/release-notes/rl-unstable.xml

@@ -92,6 +92,17 @@ was accordingly renamed to <literal>electron</literal>
   </para>
 </listitem>
 
+<listitem>
+  <para>
+    The VirtualBox host and guest options have been moved/renamed more
+    consistently and less confusing to be now found in
+    <literal>virtualisation.virtualbox.host.*</literal> instead of
+    <literal>services.virtualboxHost.*</literal> and
+    <literal>virtualisation.virtualbox.guest.*</literal> instead of
+    <literal>services.virtualboxGuest.*</literal>.
+  </para>
+</listitem>
+
 <listitem>
   <para>
     Haskell packages can no longer be found by name, i.e. the commands