mac_nvram: QOM'ify MacIO NVRAM

It was not qdev'ified before. Turn it into a SysBusDevice and
initialize it via static properties.

Prepare Old World specific MacIO state and embed the NVRAM state there.

Drop macio_nvram_setup_bar() in favor of sysbus_mmio_map() or
direct use of Memory API.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
