Stage one of multipass suspend/resume

Summary:
Add the beginnings of multipass suspend/resume, by introducing
BUS_SUSPEND_CHILD/BUS_RESUME_CHILD, and move the PCI driver to this.

Reviewers: jhb

Reviewed By: jhb

Differential Revision: https://reviews.freebsd.org/D590


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272013 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f