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 <balrogg@gmail.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>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

hw/sd/sdhci.c

@@ -1297,6 +1297,8 @@ static void sdhci_sysbus_class_init(ObjectClass *klass, void *data)
     dc->vmsd = &sdhci_vmstate;
     dc->props = sdhci_properties;
     dc->realize = sdhci_sysbus_realize;
+    /* Reason: instance_init() method uses drive_get_next() */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo sdhci_sysbus_info = {