hw/ppc/spapr: Fix boot path of usb-host storage devices

When passing through an USB storage device to a pseries guest, it
is currently not possible to automatically boot from the device
if the "bootindex" property has been specified, too (e.g. when using
"-device nec-usb-xhci -device usb-host,hostbus=1,hostaddr=2,bootindex=0"
at the command line). The problem is that QEMU builds a device tree path
like "/pci@800000020000000/usb@0/usb-host@1" and passes it to SLOF
in the /chosen/qemu,boot-list property. SLOF, however, probes the
USB device, recognizes that it is a storage device and thus changes
its name to "storage", and additionally adds a child node for the
SCSI LUN, so the correct boot path in SLOF is something like
"/pci@800000020000000/usb@0/storage@1/disk@101000000000000" instead.
So when we detect an USB mass storage device with SCSI interface,
we've got to adjust the firmware boot-device path properly that
SLOF can automatically boot from the device.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1354177
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
