acpi_piix4: expose no_hotplug attribute via i/o port

Expose no_hotplug attribute via I/O port, so ACPI BIOS can indicate
removability status to guest OS.

An updated seabios is required to make use of this feature (seabios.git
commit ID 3c241edf3d7ef29c21).

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Tested-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
