xenlight: use PHY backend when adding an empty CD-ROM and not using qdisk

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
xenlight: add block-xl script

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #25 from robhoes/master

CA-117409: xenopsd: fix CD-ROM handling