* Move `OS.Clock` out to a toplevel `Clock` module (the `mirage-clock` package).
* Move `OS.Io_page` out to a toplevel `Io_page` module (the `io-page` package).
[travis] update dependent packages
Add (untested) mir-xen-build script to link the Xen kernel from an output-obj
[unix] Remove `tuntap` stubs from Unix module; they are in `ocaml-tuntap` now.
remove tap stubs
[unix] remove blkif, console and devices modules in preparation