One extra change is to define QED_DEFAULT_CLUSTER_SIZE = 65536 instead
of 64 * 1024; because:
according to existing create_options, "cluster size" has default value =
QED_DEFAULT_CLUSTER_SIZE, after switching to create_opts, this has to be
stringized and set to .def_value_str. That is,
  .def_value_str = stringify(QED_DEFAULT_CLUSTER_SIZE),
so the QED_DEFAULT_CLUSTER_SIZE could not be a expression.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
Signed-off-by: Chunyan Liu <cyliu@suse.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>