bhyve: export funcs for read/write pci config

Export functions for reading and writing the pci config space from passthru
device to be used by other devices.
This is required for lpc devices to set their vendor/device ids to their
physical values.
Otherwise, GPU passthrough for integrated Intel GPUs won't work properly.

Differential Revision: https://reviews.freebsd.org/D33769
Reviewed by: markj
Sponsored by: Beckhoff Automation GmbH & Co. KG
MFC after: 1 month