bhyve: use assert for missing TPM version

The TPM version config node should always be set. If it's not set,
there's a bug in our code. An assertion is the correct way to check for
this.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40558