ocaml: Fix thread safety of strings in bindings (RHBZ#604691).

ocaml: Fix thread safety of strings in bindings (RHBZ#604691).

There's a thread safety issue with the current OCaml bindings which
is well explained in the bug report:

https://bugzilla.redhat.com/show_bug.cgi?id=604691

This commit fixes the safety issue by copying strings temporarily
before releasing the thread lock.  Updated code looks like this:

  char *filename = guestfs_safe_strdup (g, String_val (filenamev));
  int r;

  caml_enter_blocking_section ();
  r = guestfs_add_drive_ro (g, filename);
  caml_leave_blocking_section ();
  free (filename);
  if (r == -1)
    ocaml_guestfs_raise_error (g, "add_drive_ro");

Also included is a regression test.