hw/cpu/cluster: Mark the cpu-cluster device with user_creatable = false

hw/cpu/cluster: Mark the cpu-cluster device with user_creatable = false

The device can not be instantiated by the user and QEMU currently
aborts when you try to use it:

$ x86_64-softmmu/qemu-system-x86_64 -device cpu-cluster
qemu-system-x86_64: hw/cpu/cluster.c:73: cpu_cluster_realize:
 Assertion `cbdata.cpu_count > 0' failed.
Aborted (core dumped)

Since this is an internal device only, mark it with user_creatable = false.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1549371525-29899-1-git-send-email-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>