ocaml: evtchn+xc bindings: use libxenctrl and libxenguest

ocaml: evtchn+xc bindings: use libxenctrl and libxenguest

Now that tools/libxc is licensed under LGPL I don't think there is any need for
an LGPL reimplementation under tools/ocaml.

For the most part the conversion to the up-to-date libxc API (xc_lib.c
essentially implemented the same interface as an older libxc) was pretty
automatic. There are some functions which appear to no longer exist in libxc
which I therefore simply removed the bindings for and a small number of
interfaces which had changed.

Many of the functions bound by the stubs have no in-tree users (which I think
is fine for a language binding) so I have no way to confirm correctness other
than by eye. I was however able to confirm that oxenstored still worked and to
build a XCP toolstack which could successfully start a PV guest.

Uses the new XC_OPENFLAG_NON_REENTRANT option to avoid potential conflicts
between pthreads and the ocaml runtime.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Vincent Hanquez <Vincent.Hanquez@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>