hw/arm/virt: fix cpu object reference leak

object_new(FOO) returns an object with ref_cnt == 1
and following
  object_property_set_bool(cpuobj, true, "realized", NULL)
set parent of cpuobj to '/machine/unattached' which makes
ref_cnt == 2.

Since machvirt_init() doesn't take ownership of cpuobj
returned by object_new() it should explicitly drop
reference to cpuobj when dangling pointer is about to
go out of scope like it's done pc_new_cpu() to avoid
object leak.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-id: 1487253461-269218-1-git-send-email-imammedo@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
