hw/arm/highbank: Default -drive to if=ide instead of if=scsi

These machines have no onboard SCSI HBA, and no way to plug one.
-drive if=scsi therefore cannot work.  They do have an onboard IDE
controller (sysbus-ahci), but fail to honor if=ide.

Change their default to if=ide, and add a TODO comment on what needs
to be done to actually honor -drive if=ide.

Cc: Rob Herring <robh@kernel.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-arm@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1487153147-11530-6-git-send-email-armbru@redhat.com>