Rename accel.softmmu -> accel.system in file paths
and the register_types() method.

Rename sysemu_stubs_ss -> system_stubs_ss in meson
following the pattern used on other source set names.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231004090629.37473-7-philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>