boot: Fix boot order for SCSI target, lun > 9

We identify devices by their Open Firmware device paths.  The path
component for the logical unit on a bus is incorrect:
bootprio_find_scsi_device() and bootprio_find_usb() format target
(a.k.a. SCSI ID) and lun in decimal, while QEMU uses hexadecimal.
Bootorder list entries with target, lun > 9 aren't found (lucky case),
or attributed to the wrong logical unit (unlucky case).

The relevant spec[*] agrees with QEMU (and OVMF, for that matter).
Change %d to %x.

No actual impact on USB, because QEMU only uses LUN 0 there.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1096560

[*] Open Firmware Recommended Practice: SCSI-3 Parallel Interface,
Version 1, Section 3.1 Physical Address Formats and Representations
http://www.openfirmware.org/1275/practice/spi/spi1_0.ps
IEEE Standard for Boot (Initialization Configuration) Firmware: Core
Requirements and Practices, IEEE Std 1275-1994, Annex E SCSI host
adapter package class, section E.2.1 Physical address formats and
representations

Signed-off-by: Markus Armbruster <armbru@redhat.com>