Add missing fields in DriverBlockDevice

Pylint reports the following, let's clean them up
["Instance of 'DriverBlockDevice' has no '_fields' member", ""]
["Instance of 'DriverBlockDevice' has no '_legacy_fields' member", ""]

Change-Id: Iaf56dc35ce5ad963a6fb50d6641b203409ca6798
Closes-Bug: #1214377
Merge "Add missing fields in DriverBlockDevice"