Fix Bug: Cannot get partuuid from loop device

File: lib/disk.py

When installing on a loopback device (a.k.a loop device), function
Filesystem.partuuid_to_index() crashes with a JSON parsing error.

REASON

1) For loop devices, the property BlockDevice.device returns the
   actual image file (back-file) of the loop device instead of the
   /dev/X device.

2) Function Filesystem.partuuid_to_index() executes `lsblk --json`
   against BlockDevice.device .

3) `lsblk` fails and prints the error "not a block device" to stderr.
   This causes the output to not be valid JSON.

4) Code crashes when JSON parser tries to parse the output.

SOLUTION

- Make sure property BlockDevice.device only returns a valid block
  device.

- Create new function BlockDevice.device_or_backfile that mimics
  the present behaviour of BlockDevice.device.

- Use BlockDevice.device_or_backfile in function
  BlockDevice.__repr__().

SOLUTION REASONING

I can only see one reason behind BlockDevice.device returning
the back-file of a loop device, and that is to show the back-file
to the user (instead of /dev/X) when printing the string
representation of a BlockDevice.

All other parts of the code can use the /dev/X file just fine.

And IMO it makes more sense that a property named `device` only
returns devices, and not normal files.

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