devices: Use BFS to get downstream devices from PciBus
devices: Use BFS to get downstream devices from PciBus

It guarantees that a PCI switch always occurs before its downstream
devices.

BUG=b:194390621
TEST=tools/presubmit

Change-Id: I41f76ca2f99c2cf9b26c49081e580f75689b0c1d
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3844814
Auto-Submit: Victor Ding <b4a7cab2b204d4052366cc26438aa3555eb3d577@chromium.org>
Reviewed-by: Tinghao Zhang <cc7080d957656df4a0515a80b65a58b0eaab2dde@intel.corp-partner.google.com>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Tested-by: Victor Ding <b4a7cab2b204d4052366cc26438aa3555eb3d577@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm