sysbus: Make devices picking up backends unavailable with -device

sysbus: Make devices picking up backends unavailable with -device

Device models aren't supposed to go on fishing expeditions for
backends.  They should expose suitable properties for the user to set.
For onboard devices, board code sets them.

A number of sysbus devices pick up block backends in their init() /
instance_init() methods with drive_get_next() instead: sl-nand,
milkymist-memcard, pl181, generic-sdhci.

Likewise, a number of sysbus devices pick up character backends in
their init() / realize() methods with qemu_char_get_next_serial():
cadence_uart, digic-uart, etraxfs,serial, lm32-juart, lm32-uart,
milkymist-uart, pl011, stm32f2xx-usart, xlnx.xps-uartlite.

All these mistakes are already marked FIXME.  See the commit that
added these FIXMEs for a more detailed explanation of what's wrong.

Fortunately, only machines ppce500 and pseries-* support -device with
sysbus devices, and none of the devices above is supported with these
machines.

Set cannot_instantiate_with_device_add_yet to preserve our luck.

Cc: Andrzej Zaborowski <[email protected]>
Cc: Peter Crosthwaite <[email protected]>
Cc: Antony Pavlov <[email protected]>
Cc: "Edgar E. Iglesias" <[email protected]>
Cc: Michael Walle <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>