Add a function for verified erase.

get_device_type: return the type (USB, ATA, MMC, OTHER)
of a block device.

BUG=chrome-os-partner:24465
TEST=Run locally against ATA and USB device.

Change-Id: I005697222a90cb358d3631c3415e3062745ae660
Reviewed-on: https://chromium-review.googlesource.com/194326
Reviewed-by: Puthikorn Voravootivat <puthik@chromium.org>
Commit-Queue: Gwendal Grignou <gwendal@chromium.org>
Tested-by: Gwendal Grignou <gwendal@chromium.org>