hw/virtio/virtio-mem: Use qemu_ram_get_fd() helper

Avoid accessing RAMBlock internals, use the provided
qemu_ram_get_fd() getter to get the file descriptor.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230524093744.88442-7-philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

hw/virtio/virtio-mem.c

@@ -135,7 +135,7 @@ static bool virtio_mem_has_shared_zeropage(RAMBlock *rb)
      * anonymous RAM. In any other case, reading unplugged *can* populate a
      * fresh page, consuming actual memory.
      */
-    return !qemu_ram_is_shared(rb) && rb->fd < 0 &&
+    return !qemu_ram_is_shared(rb) && qemu_ram_get_fd(rb) < 0 &&
            qemu_ram_pagesize(rb) == qemu_real_host_page_size();
 }
 #endif /* VIRTIO_MEM_HAS_LEGACY_GUESTS */