iotests: handle TypeError for Python 3 in test 242

The data type for bytes in Python 3 differs from the one in Python 2.
The type cast that is compatible with both versions was applied.
Signed-off-by: NNir Soffer <nsoffer@redhat.com>
Signed-off-by: NAndrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reported-by: NKevin Wolf <kwolf@redhat.com>
Message-Id: <1551197495-24425-1-git-send-email-andrey.shinkevich@virtuozzo.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

tests/qemu-iotests/242

@@ -20,6 +20,7 @@
 
 import iotests
 import json
+import struct
 from iotests import qemu_img_create, qemu_io, qemu_img_pipe, \
     file_path, img_info_log, log, filter_qemu_io
 
@@ -64,10 +65,11 @@ def write_to_disk(offset, size):
 def toggle_flag(offset):
     with open(disk, "r+b") as f:
         f.seek(offset, 0)
-        c = f.read(1)
-        toggled = chr(ord(c) ^ bitmap_flag_unknown)
+        # Read one byte in a way compatible with Python 2
+        flags = struct.unpack("B", f.read(1))
+        toggled = flags[0] ^ bitmap_flag_unknown
         f.seek(-1, 1)
-        f.write(toggled)
+        f.write(struct.pack("B", toggled))
 
 
 qemu_img_create('-f', iotests.imgfmt, disk, '1M')