hw/arm/msf2-som: Exit when the cpu is not the expected one

hw/arm/msf2-som: Exit when the cpu is not the expected one

This machine correctly defines its default_cpu_type to cortex-m3
and report an error if the user requested another cpu_type,
however it does not exit, and this can confuse users trying
to use another core:

  $ qemu-system-arm -M emcraft-sf2 -cpu cortex-m4 -kernel test-m4.elf
  qemu-system-arm: This board can only be used with CPU cortex-m3-arm-cpu
  [output related to M3 core ...]

The CPU is indeed a M3 core:

  (qemu) info qom-tree
  /machine (emcraft-sf2-machine)
    /unattached (container)
      /device[0] (msf2-soc)
        /armv7m (armv7m)
          /cpu (cortex-m3-arm-cpu)

Add the missing exit() call to return to the shell.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-id: 20190617160136.29930-1-philmd@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>