Split the block device creation in two parts

Some callers of _CreateBlockDev need recursive behaviour, but not all.
The replace secondary first creates (manually) new LVs to ensure storage
is there, and then it creates the new DRBD. At this point, we need a
non-recursive call so that the LVs are not needlessly re-created.

This patch splits the single device creation into a separate function,
so that LUReplaceDisks can use it.

Reviewed-by: ultrotter