hw/arm/virt: Hide host CPU model for tcg

hw/arm/virt: Hide host CPU model for tcg

The 'host' CPU model isn't available until KVM or HVF is enabled.
For example, the following error messages are seen when the guest
is started with option '-cpu cortex-a8' on tcg after the next commit
is applied to check the CPU type in machine_run_board_init().

  ERROR:../hw/core/machine.c:1423:is_cpu_type_supported: \
  assertion failed: (model != NULL)
  Bail out! ERROR:../hw/core/machine.c:1423:is_cpu_type_supported: \
  assertion failed: (model != NULL)
  Aborted (core dumped)

Hide 'host' CPU model until KVM or HVF is enabled. With this applied,
the valid CPU models can be shown.

  qemu-system-aarch64: Invalid CPU type: cortex-a8
  The valid types are: cortex-a7, cortex-a15, cortex-a35, \
  cortex-a55, cortex-a72, cortex-a76, cortex-a710, a64fx, \
  neoverse-n1, neoverse-v1, neoverse-n2, cortex-a53,      \
  cortex-a57, max

Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231204004726.483558-6-gshan@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>