libxl: prepare environment for domcreate_stream_done

libxl: prepare environment for domcreate_stream_done

The function domcreate_bootloader_done may branch early to
domcreate_stream_done, in case some error occoured. Here srs->dcs will be
NULL, which leads to a crash.

It is unclear what the purpose of that backpointer is. Perhaps it can be
removed, and domcreate_stream_done could use CONTAINER_OF.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Wei Liu <wei.liu2@citrix.com>
[ wei: fold in comment required by Ian ]
Signed-off-by: Wei Liu <wei.liu2@citrix.com>