bhyve: make use of helper to read PCI IDs from bhyve config

For compatibilty reasons, the old config values are still supported.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38403

(cherry picked from commit b72e06b13ee08b94a3e74ac2473e889aeb7a3d1a)