vm/windows: Split install into several stages.

These stages are in particular:

 * Install of the bare Windows VM with Cygwin and shut down.
 * Boot up the same VM again without the installation media and dump the
   VMs memory to state.gz.
 * Resume from state.gz and build whatever we want to build.

Every single stage involves a new "controller", which is more like an
abstraction on the Nix side that constructs the madness described in
276b72fb

.

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