hw: Mark devices picking up block backends actively FIXME

Drives defined with if!=none are for board initialization to wire up.
Board code calls drive_get() or similar to find them, and creates
devices with their qdev drive properties set accordingly.

Except a few devices go on a fishing expedition for a suitable backend
instead of exposing a drive property for board code to set: they call
driver_get() or drive_get_next() in their realize() or init() method
to implicitly connect to the "next" backend with a certain interface
type.

Picking up backends that way works when the devices are created by
board code.  But it's inappropriate for -device or device_add.  Not
only is this inconsistent with how the other block device models work
(they connect to a backend explicitly identified by a "drive"
property), it breaks when the "next" backend has been picked up by the
board already.

Example:

    $ qemu-system-arm -S -M connex -pflash flash.img -device ssi-sd
    Aborted (core dumped)

Mark them with suitable FIXME comments.

Cc: Andrzej Zaborowski <balrogg@gmail.com>
Cc: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Cc: "Andreas Färber" <andreas.faerber@web.de>
Cc: Michael Walle <michael@walle.cc>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
