Machine types cubieboard, xlnx-ep108, xlnx-zcu102 have an onboard AHCI
controller, but neglect to set their MachineClass member
units_per_default_bus = 1.  This permits -drive if=ide,unit=1, which
makes no sense for AHCI.  It also screws up index=N for odd N, because
it gets desugared to unit=1,bus=N/2

Doesn't really matter, because these machine types fail to honor
-drive if=ide.  Add the missing units_per_default_bus = 1 anyway,
along with a TODO comment on what needs to be done for -drive if=ide.

Also set block_default_type = IF_IDE explicitly.  It's currently the
default, but the next commit will change it to something more
sensible, and we want to keep the IF_IDE default for these three
machines.  See also the previous commit.

Cc: Beniamino Galvani <b.galvani@gmail.com>
Cc: Alistair Francis <alistair.francis@xilinx.com>
Cc: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-arm@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Alistair Francis <alistair.francis@xilinx.com>
Message-Id: <1487153147-11530-3-git-send-email-armbru@redhat.com>