fs: debug VDI.similar_content
fs: debug VDI.similar_content

Distance should be computed by symmetric_difference of chains, not
intersections. We should ignore all disks whose intersection is empty
because these are completely unrelated disks.

djs55/message-switch,xapi-project/message-switch,xapi-project/message-switch,djs55/message-switch-1,djs55/message-switch-1,djs55/message-switch,xapi-project/message-switch,djs55/message-switch,djs55/message-switch-1