Conditionally compile sparse_dd

This needs a bunch of libraries which are only present in an
environment with xapi-project/opam-repo-dev available.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update CHANGES for 0.6.5

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #7 from djs55/separate-sparse_dd

Separate sparse dd