Instance Creation: generate nics earlier

We want the real nic to be shown to the hooks and the allocators, so
we'll generate them in CheckPrereq. We also write a comment about the
race condition we generate. This race condition existed even before, so
moving this generation will just lenghen it a bit. A separate patch
mitigates its effects.

This patch also adds an ENDIF comment for a very long if, and removes a
double empty line inside the CheckPrereq function of LUCreateInstance.

Reviewed-by: iustin