Add sparse_dd to the xapi package

This is needed by VDI.copy

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
... and add a changelog entry for sparse_dd

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #147 from djs55/add-sparse-dd

Add sparse_dd to the xapi package