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

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 <[email protected]>
Cc: "Edgar E. Iglesias" <[email protected]>
Cc: Alistair Francis <[email protected]>
Cc: [email protected]
Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Message-Id: <1487153147[email protected]>