Update ocaml-vhd to 0.6.1

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update xapi to 1.9.38

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update ffs to 0.9.22

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Correct changelog typo

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #302 from djs55/vdi-copy

VDI.clone, VDI.copy fixes