pvscsi: include stddef in xl_cmdimpl.c to get offsetof

Signed-off-by: Olaf Hering <olaf@aepfle.de>
pvscsi: simplify sysfs parsing in parse_vscsi_config

xl is single threaded, prefer readdir over readdir_r
reorder local variables
adapt comments to upstream coding style

Signed-off-by: Olaf Hering <olaf@aepfle.de>
pvscsi: implement simple scsi-detach

Signed-off-by: Olaf Hering <olaf@aepfle.de>