hw/isa/pc87312: Mark the device with user_creatable = false

QEMU currently aborts if you try to use the device at the command
line:

$ ppc64-softmmu/qemu-system-ppc64 -S -machine prep -device pc87312
Unexpected error in qemu_chr_fe_init() at chardev/char-fe.c:222:
qemu-system-ppc64: -device pc87312: Device 'parallel0' is in use
Aborted (core dumped)

It uses parallel_hds in its realize function, so I can not be
instantiated by the user again.
Signed-off-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NHervé Poussineau <hpoussin@reactos.org>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/isa/pc87312.c

@@ -386,6 +386,8 @@ static void pc87312_class_init(ObjectClass *klass, void *data)
     dc->reset = pc87312_reset;
     dc->vmsd = &vmstate_pc87312;
     dc->props = pc87312_properties;
+    /* Reason: Uses parallel_hds[0] in realize(), so it can't be used twice */
+    dc->user_creatable = false;
 }
 
 static const TypeInfo pc87312_type_info = {