isa: Clean up use of cannot_instantiate_with_device_add_yet

Drop it when there's no obvious reason why device_add could not work.
Else keep and document why.

* isa-fdc: drop

* i8042: drop, even though its I/O base is hardcoded (because you
  could conceivably still add one to a board that has none), and even
  though PC board code wires up the A20 line (because that wiring is
  optional)

* port92: keep because it needs additional wiring by port92_init()

* mc146818rtc: keep because it needs to be wired up by rtc_init()

* m48t59_isa: keep because needs to be wired up by m48t59_init_isa()

* isa-pit, kvm-pit: keep (in their abstract base pic-common) because
  the PIT needs additional wiring by board code, depending on HPET
  presence

* pcspk: keep because of pointer property pit, and because realize
  sets global pcspk_state

* vmmouse: keep because of pointer property ps2_mouse

* vmport: keep because realize sets global port_state

* isa-i8259, kvm-i8259: keep (in their abstract base pic-common),
  because the PICs' IRQ input lines are set up by board code, and the
  wiring of the slave to the master is hard-coded in device model code

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>

hw/input/pckbd.c

@@ -522,7 +522,6 @@ static void i8042_class_initfn(ObjectClass *klass, void *data)
     DeviceClass *dc = DEVICE_CLASS(klass);
 
     dc->realize = i8042_realizefn;
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
     dc->vmsd = &vmstate_kbd_isa;
 }