[Libguestfs] [PATCH 2/3] Convert source so it can be compiled with OCaml '-safe-string' option.

OCaml 4.02 introduced the 'bytes' type, a mutable string intended to replace the existing 'string' type for those cases where the byte array can be mutated. In future the 'string' type will become immutable. This is not the default now, but it can be forced using the '-safe-string' compile option. I tested this on Fedora 24 (OCaml 4.02) & RHEL 7 (OCaml 4.01). Rich.