x86: Support multiple PCI MCFG regions

In particular, this enables support for PCI config access for domains
(segments) other than 0.

Reported by: cperciva
Tested by: cperciva (m7i.metal-48xl AWS instance)
Reviewed by: imp
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D42828

(cherry picked from commit f54a3890b1f419d4a9edc99000efef52b9071b8f)