Add ACPI based PCI hotplug library with bridge hotplug
support.
Design
   - each bus gets assigned "bsel" property.
   - ACPI code writes this number
     to a new BNUM register, then uses existing
     UP/DOWN registers to probe slot status;
     to eject, write number to BNUM register,
     then slot into existing EJ.

The interface is actually backwards-compatible with
existing PIIX4 ACPI (though not migration compatible).

This is split out from PIIX4 codebase so we can
reuse it for Q35 as well.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>