fw_cfg: ignore suffixes in the bootdevice list dependent on machine class

fw_cfg: ignore suffixes in the bootdevice list dependent on machine class

For the older machines (such as Mac and SPARC) the DT nodes representing
bootdevices for disk nodes are irregular for mainly historical reasons.

Since the majority of bootdevice nodes for these machines either do not have a
separate disk node or require different (custom) names then it is much easier
for processing to just disable all suffixes for a particular machine.

Introduce a new ignore_boot_device_suffixes MachineClass property to control
bootdevice suffix generation, defaulting to false in order to preserve
compatibility.

Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20180810124027.10698-1-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>