bhyve: add hook for PCI header of passthru devices

Most register of the PCI header are either constant values or require
emulation anyway. The command and status register are the only exception which
require hardware access. So, we're adding an emulation handler for all
other register.

As this emulation handler will be reused by some future features like
GPU passthrough, we directly export it.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33010