Second iteration over backend.BlockdevWipe

This patch now uses dd entirely to wipe the disk, make it
much easier to wipe in blocks so we can give interactive feedback
about the status.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>