Tails Installer fails to list USB stick with isohybrid Tails installed

[gui:422] drives: {'/dev/sda': {'size': 4007657472L, 'is_optical': False, 'vendor': u'Verbatim', 'fsversion': '', 'parent': None, 'bootable': False, 'mount': None, 'parent_size': None, 'mounted_partitions': set([]), 'free': None, 'label': u'TAILS_0.21_-_20131027', 'fstype': 'iso9660', 'parent_udi': None, 'device': '/dev/sda', 'model': u'STORE N GO', 'udi': '/org/freedesktop/UDisks/devices/sda', 'uuid': ''}} Traceback (most recent call last): File "/usr/lib/pymodules/python2.6/dbus/connection.py", line 586, in msg_reply_handler reply_handler(*message.get_args_list(get_args_opts)) File "/usr/lib/python2.6/dist-packages/liveusb/creator.py", line 724, in handle_reply callback() File "/usr/lib/python2.6/dist-packages/liveusb/gui.py", line 439, in add_devices 'size': _format_bytes_in_gb(info['parent_size']) File "/usr/lib/python2.6/dist-packages/liveusb/utils.py", line 100, in _format_bytes_in_gb return '%0.1f GB' % (value / 10.09) TypeError: unsupported operand type(s) for /: 'NoneType' and 'float'

I guess this bug was introduced in the liveusb GUI improvements, so tentatively assigning to Alan.

Feature Branch: liveusb:bugfix/list_isohybrid

Original created by @intrigeri on 6462 (Redmine)