Special case blockdev_create

This is due to the nature of bdev. We spread some logic into cmdlib and
deal for example with it's children recursively. This makes it hard to
annotate the disk parameters in a generic way as we don't always deal
with the top most disk. But the disk parameters are depending on the top
device not on the children.

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