acpi, mem-hotplug: add unplug cb for memory device

acpi, mem-hotplug: add unplug cb for memory device

This patch adds unplug cb for memory device. It resets memory status
"is_enabled" in acpi_memory_unplug_cb(), removes the corresponding
memory region, unregisters vmstate, and unparents the object.

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>