More informations coming:

Vhd attached with:

xl block-attach 2 tap:vhd:/cloud/data2/machines/vm2.vhd xvdb w

Strace results of detach attempt with xl block-detach in attached  file 
xl_vhd_detach.txt
(seems to receive an OK but it doesn't detach anything)

Strace results of detach attempt with xm block-detach in attached file 
xm_vhd_detach.txt
(doesn't seem to know anything about the device existence)

Hope this can help for debugging the things out :)

Cheers,
Sébastien