Update to mirage 4.5.0 and improve netvm features

• Fix an incorrect free memory estimation (fix in mirage/ocaml-solo5#135 @palainp)
• Update to mirage 4.5.0, allowing openBSD to be used as netvm (#146 reported by @Szewcson), and recover from a netvm change (#156 reported by @xaki23) (#178 @palainp)