fd_send and fd_recv need update

[mseri]

The two function used here
https://github.com/xapi-project/stdext/blob/master/lib/xapi-stdext-unix/unixext.ml#L660-L661
are currently taking a string as buffer. This is currently working also on ocaml-4.06.0 but it's not guaranteed to work in the future. We should change the ocaml-fd-send-recv interface, and as a consequence this interface, to use bytes instead. I believe the C code will stay unchanged, it is only an interface issue