bench: each VBD has a distinct devid (previously they were all 0)
bench: each VBD has a distinct devid (previously they were all 0)

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

mirage/ocaml-xenstore-server,djs55/ocaml-xenstore-server,mirage/ocaml-xenstore-server