tpm2-util: tighten rules on the nvindex handle range we allocate from

tpm2-util: tighten rules on the nvindex handle range we allocate from

Let's follow the conventions set by "Registry of Reserved TPM 2.0 Handles
and Localities" and only allocate nvindex currently not assigned to any
vendor.

For details see:

https://trustedcomputinggroup.org/resource/registry/

Section 2.2