BDM class and transformation functions

This patch adds the utility functions that will help us switch
between versions of block device mapping data.

In order to accomplish this, and to have a more structured approach to
dealing with block device mapping data in the future, this patch
introduces a BlockDeviceDict class which is a thin wrapper around a
standard dict that makes it easier to reason about which fields are
present in the dictionary, and introduces said conversion routines.

blueprint: improve-block-device-handling

Change-Id: I9370333059b8c9aaf92010470b8475a913d329b2
Merge "BDM class and transformation functions"