ocaml/xenctrl: Make failwith_xc() thread safe

ocaml/xenctrl: Make failwith_xc() thread safe

The static error_str[] buffer is not thread-safe, and 1024 bytes is
unreasonably large.  Reduce to 256 bytes (which is still much larger than any
current use), and move it to being a stack variable.

Also, propagate the Noreturn attribute from caml_raise_with_string().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Dave Scott <Dave.Scott@eu.citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: David Scott <dave.scott@citrix.com>