Associate device_t objects with ACPI handles via PCI_CHILD_ADDED().

Previously, the ACPI PCI bus driver did a single pass over the devices in
the namespace that were a child of a given PCI bus to as

Associate device_t objects with ACPI handles via PCI_CHILD_ADDED().

Previously, the ACPI PCI bus driver did a single pass over the devices in
the namespace that were a child of a given PCI bus to associate the
PCI bus-enumerated device_t devices with the corresponding ACPI handles.
However, this meant that handles were only established at runtime for devices
found during the initial PCI bus scan.

PCI_IOV adds devices that show up after the initial PCI bus scan, and coming
changes to add a bus rescan can also add devices after the initial scan.

This change adds a pci_child_added() callback to the ACPI PCI bus that walks
the namespace to find the ACPI handle for each device that is added. Using
a callback means that the handle is correctly set for any device no matter
how it is added (initial scan, IOV, or a bus rescan).

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