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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 7e14be0b0717105f4b3b8c62df82a1e883d8ebb6)