vm/windows: Move creating SSH key into install/.

This SSH key is specifically only for accessing the installed Cygwin
within the Windows VM, so we only need to expose the private key. Yes,
you heard right, the private key. It's not security-relevant because the
machine is completely read-only, only exposed to the filesystem and
networking is not available.

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