hw/ppc/spapr: Fix virtio-scsi bootindex handling for LUNs >= 256

LUNs >= 256 have to be encoded with the so-called "flat space
addressing method" for virtio-scsi, where an additional bit has to
be set. SLOF already took care of this with the following commit:

 https://git.qemu.org/?p=SLOF.git;a=commitdiff;h=f72a37713fea47da
 (see https://bugzilla.redhat.com/show_bug.cgi?id=1431584 for details)

But QEMU does not use this encoding yet for device tree paths
that have to be handed over to SLOF to deal with the "bootindex"
property, so SLOF currently fails to boot from virtio-scsi devices
with LUNs >= 256 in the right boot order. Fix it by using the bit
to indicate the "flat space addressing method" for LUNs >= 256.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
