hw/virtio/meson: Rename softmmu_virtio_ss[] -> system_virtio_ss[]

hw/virtio/meson: Rename softmmu_virtio_ss[] -> system_virtio_ss[]

Similarly to commit de6cd7599b ("meson: Replace softmmu_ss
-> system_ss"), rename the virtio source set common to all
system emulation as 'system_virtio_ss[]'. This is clearer
because softmmu can be used for user emulation.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230710100510.84862-1-philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>