libxc/libxl/python/xenstat/ocaml: Use new XEN_VERSION hypercall

libxc/libxl/python/xenstat/ocaml: Use new XEN_VERSION hypercall

We change the xen_version libxc code to use the new hypercall.
Which of course means every user in the code base has to
be changed over.

It is important to note that the xc_version_op has a different
return semantic than the previous one. It returns negative
values on error (like the old one), but it also returns
an positive value on success (unlike the old one). The positive
value is the number of bytes copied in.

Note that both Ocaml and xenstat use tabs instead of four
spaces so they look quite odd.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com> [for the Ocaml stubs]
Acked-by: George Dunlap <george.dunlap@eu.citrix.com> [xenctx bits]
Acked-by: Wei Liu <wei.liu2@citrix.com>