hw: Mark devices picking up char backends actively FIXME

Character devices defined with -serial and -parallel are for board
initialization to wire up.  Board code examines serial_hds[] and
parallel_hds[] to find them, and creates devices with their qdev
chardev properties set accordingly.

Except a few devices go on a fishing expedition for a suitable backend
instead of exposing a chardev property for board code to set: they use
serial_hds[] (often via qemu_char_get_next_serial()) or parallel_hds[]
in their realize() or init() method to connect to a backend.

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 it inconsistent with how the other characrer device models
work (they connect to a backend explicitly identified by a "chardev"
property), it breaks when the backend has been picked up by the board
or a previous -device / device_add already.

Example:

    $ qemu-system-ppc64 -M bamboo -S -device i82378 -device pc87312 -device pc87312
    qemu-system-ppc64: -device pc87312: Property 'isa-parallel.chardev' can't take value 'parallel0', it's in use

Mark them with suitable FIXME comments.

Cc: Li Guang <lig.fnst@cn.fujitsu.com>
Cc: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Cc: Antony Pavlov <antonynpavlov@gmail.com>
Cc: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Cc: Michael Walle <michael@walle.cc>
Cc: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Cc: "Andreas Färber" <andreas.faerber@web.de>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
