acpi: add hardware implementation for memory hot unplug

acpi: add hardware implementation for memory hot unplug

- implements QEMU hardware part of memory hot unplug protocol
  described at "docs/spec/acpi_mem_hotplug.txt"
- handles memory remove notification event
- handles device eject notification

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Zhu Guihua <zhugh.fnst@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>