pci: Add an ioctl to perform I/O to BARs

This is useful for bhyve, which otherwise has to use /dev/io to handle
accesses to I/O port BARs when PCI passthrough is in use.

Reviewed by: imp, kib
Discussed with: jhb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31307