zdtm: open notify file descriptors in a binary mode

  Send pre-dump notify to 36
  Traceback (most recent call last):
    File "zdtm.py", line 2161, in <module>
      do_run_test(tinfo[0], tinfo[1], tinfo[2], tinfo[3])
    File "zdtm.py", line 1549, in do_run_test
      cr(cr_api, t, opts)
    File "zdtm.py", line 1264, in cr
      test.pre_dump_notify()
    File "zdtm.py", line 490, in pre_dump_notify
      fdin.write(struct.pack("i", 0))
  TypeError: write() argument 1 must be unicode, not str

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>