virtio_fs: correct tags for config space fields

Since fs is a modern-only device,
tag config space fields as having little endian-ness.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>