Implement VDI.{compose,similar_content}

Note VDI.similar_content is currently a stub which returns []

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Remove dead code

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update ChangeLog

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #18 from djs55/similar-content

Implement VDI.compose, VDI.similar_content