vmdk: Implement .bdrv_get_info() This will return cluster_size and needs_compressed_writes to caller, if all the extents have the same value (or there's only one extent). Otherwise return -ENOTSUP. cluster_size is only reported for sparse formats. Signed-off-by: Fam Zheng <famz@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>