Improve Disk Detection

The previous implementation of list_block_devices used blockdev,
which would list partitions, software RAID and other devices as block devices.
By switching to lsblk, the agent can filter down to only physical block
devices, which is all the agent cares about for any of its operations.

This change adds two new fields to the BlockDevice class: model, a string of
the block devices reported model, and rotational, a boolean representing a
spinning disk (True) or a solid state disk (False). This data can be useful
for vendor hardware managers.

Change-Id: I385c3bb378c2c49385bca14a1d7efa074933becf
Closes-Bug: 1344351