qom: move CPUClass.tcg_initialize to a global

qom: move CPUClass.tcg_initialize to a global

55c3cee ("qom: Introduce CPUClass.tcg_initialize", 2017-10-24)
introduces a per-CPUClass bool that we check so that the target CPU
is initialized for TCG only once. This works well except when
we end up creating more than one CPUClass, in which case we end
up incorrectly initializing TCG more than once, i.e. once for
each CPUClass.

This can be replicated with:
  $ aarch64-softmmu/qemu-system-aarch64 -machine xlnx-zcu102 -smp 6 \
      -global driver=xlnx,,zynqmp,property=has_rpu,value=on
In this case the class name of the "RPUs" is prefixed by "cortex-r5-",
whereas the "regular" CPUs are prefixed by "cortex-a53-". This
results in two CPUClass instances being created.

Fix it by introducing a static variable, so that only the first
target CPU being initialized will initialize the target-dependent
part of TCG, regardless of CPUClass instances.

Fixes: 55c3ceef61fcf06fc98ddc752b7cce788ce7680b
Signed-off-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1510343626-25861-2-git-send-email-cota@braap.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>