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

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>