Reorder checks in instance create

This patch reorders the checks in the instance create prereq so that all
checks and normalisations that are not node-dependent are done before
the node dependent checks.

This is done so that, after the instance-related opcode parameters are
checked and fixed, we can run the allocator and compute the primary (and
any secondary) nodes, and only then proceed with node-related checks.

Reviewed-by: ultrotter