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

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

This patch adds unplug request cb for memory device, and adds the
is_removing boolean field to MemStatus. This field is used to indicate
whether the memory device in slot has been requested to be ejected.
This field is set to true in acpi_memory_unplug_request_cb().

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>