Remove constant for disk wipe block size

It is dangerous to have this block size as a global constant as that
could give the impression of it being easily changed. Doing so without
further adjustments to how “dd” is called will lead to disks not being
wiped properly.

Two additional parameter checks are added to “backend.BlockdevWipe”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>