hw/arm: Mark the "fsl,imx6" device with user_creatable = false

This device causes QEMU to abort if the user tries to instantiate it:

$ qemu-system-aarch64 -M sabrelite -smp 1,maxcpus=2 -device fsl,,imx6
Unexpected error in qemu_chr_fe_init() at chardev/char-fe.c:222:
qemu-system-aarch64: -device fsl,,imx6: Device 'serial0' is in use
Aborted (core dumped)

The device uses serial_hds[] directly in its realize function, so it
can not be instantiated again by the user.
Signed-off-by: NThomas Huth <thuth@redhat.com>
Message-id: 1509519537-6964-2-git-send-email-thuth@redhat.com
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/arm/fsl-imx6.c

@@ -440,8 +440,9 @@ static void fsl_imx6_class_init(ObjectClass *oc, void *data)
     DeviceClass *dc = DEVICE_CLASS(oc);
 
     dc->realize = fsl_imx6_realize;
-
     dc->desc = "i.MX6 SOC";
+    /* Reason: Uses serial_hds[] in the realize() function */
+    dc->user_creatable = false;
 }
 
 static const TypeInfo fsl_imx6_type_info = {