viscera: machines: return boot_disk as BlockDevice (#161)
viscera: machines: return boot_disk as BlockDevice (#161)

Trying to read `boot_disk` on a `machines` instance returns a
`TypeError`:

  {'name': 'sda', ...} is not of type (<class 'str'>, <class
  'NoneType'>)

The commit defines `boot_disk` as a `BlockDevice` instead, which returns
the expected object.
blakerouse/python-libmaas,alburnum/alburnum-maas-client