hw/virtio: Compile virtio-mem.c once

hw/virtio: Compile virtio-mem.c once

Remove unused "system/ram_addr.h" header. This file doesn't
use any target specific definitions anymore, compile it once
by moving it to system_virtio_ss[].

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20250502214551.80401-6-philmd@linaro.org>