setup-storage: preserve_lazy for LVM fails on empty disk

[Nicolas Courtel]

Hello,

Context: FAI 3.4~beta1+experimental11, disk is empty, and the 
preserve_lazy option is set for two LVM volumes.
The volume group is not created by setup-storage, pvcreate is followed 
by vgextend instead of vgcreate.

I'm pretty sure this has been working before, I think I have 
successfully used it with experimental9.
Of course, I just need to remove the preserve_lazy option to make it 
work again.

The format.log file is on http://paste.debian.net/71013/

-- 
Nicolas