hw: Default -drive to if=none instead of scsi when scsi cannot work

Block backends defined with -drive if=scsi are meant to be picked up
by machine initialization code: a suitable frontend gets created and
wired up automatically.

if=scsi drives not picked up that way can still be used with -device
as if they had if=none, but that's unclean and best avoided.  Unused
ones produce an "Orphaned drive without device" warning.

A few machine types default to if=scsi, even though they don't
actually have a SCSI HBA.  This makes no sense.  Change their default
to if=none.  Affected machines:

* aarch64/arm: realview-pbx-a9 vexpress-a9 vexpress-a15 xilinx-zynq-a9

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Cc: Alistair Francis <alistair.francis@xilinx.com>
Cc: qemu-arm@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-Id: <1487153147-11530-5-git-send-email-armbru@redhat.com>

hw/arm/realview.c

@@ -443,7 +443,6 @@ static void realview_pbx_a9_class_init(ObjectClass *oc, void *data)
 
     mc->desc = "ARM RealView Platform Baseboard Explore for Cortex-A9";
     mc->init = realview_pbx_a9_init;
-    mc->block_default_type = IF_SCSI;
     mc->max_cpus = 4;
 }
 

hw/arm/vexpress.c

@@ -752,7 +752,6 @@ static void vexpress_class_init(ObjectClass *oc, void *data)
 
     mc->desc = "ARM Versatile Express";
     mc->init = vexpress_common_init;
-    mc->block_default_type = IF_SCSI;
     mc->max_cpus = 4;
 }
 

hw/arm/xilinx_zynq.c

@@ -323,7 +323,6 @@ static void zynq_machine_init(MachineClass *mc)
 {
     mc->desc = "Xilinx Zynq Platform Baseboard for Cortex-A9";
     mc->init = zynq_init;
-    mc->block_default_type = IF_SCSI;
     mc->max_cpus = 1;
     mc->no_sdcard = 1;
 }