hw/arm/allwinner-a10: Mark the allwinner-a10 device with user_creatable = false

QEMU currently exits unexpectedly when the user accidentially
tries to do something like this:

$ aarch64-softmmu/qemu-system-aarch64 -S -M integratorcp -nographic
QEMU 2.9.93 monitor - type 'help' for more information
(qemu) device_add allwinner-a10
Unsupported NIC model: smc91c111

Exiting just due to a "device_add" should not happen. Looking closer
at the the realize and instance_init function of this device also
reveals that it is using serial_hds and nd_table directly there, so
this device is clearly not creatable by the user and should be marked
accordingly.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-id: 1503416789-32080-1-git-send-email-thuth@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>