add Xen makefile targets also
xen: add mmap_unix from 4.00.1, a dependency of Bigarray

In our config.h we switch off almost everything, including mmap.
The mmap_stubs exist but will throw an exception if called.
Merge pull request #44 from djs55/add-mmap-stubs

xen: add mmap_unix from 4.00.1, a dependency of Bigarray
update for mirage/mirage-platform#44
reformat with ocp-indent
Make the Eventchn module like the one in ocaml-xen-lowlevel-libs, updating doc.
Unified interface with UNIX bindings to libxc.
Unify Activations with ocaml-xen-lowlevel-libs.
Updated CHANGES.
[xen] Improved Io_page, implemented missing Gnt bits.
Added a Eventchn.close function for compatibility with the UNIX bindings.
added check to wait for required number of free ring slots
Merge pull request #46 from balrajsingh/master

Added check to wait for required number of ring slots before writing
Make Io_page in the UNIX backend compatible with Io_page in the Xen backend.