ide: Fix bootindex for bus_id > 9

ide: Fix bootindex for bus_id > 9

We identify devices by their Open Firmware device paths.  The encoding
of bus numbers is incorrect: idebus_get_fw_dev_path() formats them in
decimal, while SeaBIOS uses hexadecimal.  With bus number > 9, SeaBIOS
will miss the bootindex (lucky case), or apply it to another device
(unlucky case).

Bug can't bite right now: ich9-ahci has six ports, and the sysbus-ahci
created by Calxeda Highbank has just one.

Fix it anyway, by changing %d to %x.

I couldn't find an Open Firmware spec covering this.  For what it's
worth, OVMF agrees with SeaBIOS.

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