bhyve: scan PCI device functions to find host LPC

At least on some AMD devices the host LPC bridge could be located as
seperate function of another PCI device.

Fixes: f4ceaff56ddaacc151df07d2d205a2d7fcb736a8
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39310