acpi, pc: Add hotunplug request cb for pc machine.

acpi, pc: Add hotunplug request cb for pc machine.

Memory and CPU hot unplug are both asynchronous procedures.
They both need unplug request callback to initiate unplug operation.

Add unplug handler to pc machine that will be used by following
CPU and memory unplug patches.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Tang Chen <tangchen@cn.fujitsu.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>