mac_newworld: use node name instead of alias name for hd device in FWPathProvider

When using -drive to configure the hd drive for the New World machine, the node
name "disk" should be used instead of the "hd" alias.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20190307212058.4890-3-mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>