Rework the code to detect disks automatically

The user no longer needs to point to disk device nodes when initializing
a new disk or when configuring the SDK to use an existing persistent
disk. When using the command line, the only option that accepts a path
is --etc to add a configuration file to persistent storage.

A class Disks was added to handle disk detection based on the output of
lsblk. This made it possible to replace various other methods and
functions that were overlaping in functionality.

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>