devices:pcie: Remove downstream device when guest poweroff root port
devices:pcie: Remove downstream device when guest poweroff root port

When guest poweroff root port, its downstream device could be destroyed
safely, so put the downstream device into
PciDevice->get_removed_children_devices(), then the downstream device
will be removed by pci root.

BUG=b:185084350
TEST=Boot a guest and hotplug vfio pci device repeatedly

Change-Id: Icb8104a2417832864d8dbe4947895028dabb46d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2955577
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

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