streaming: coalesce copies of adjacent sectors
streaming: coalesce copies of adjacent sectors

This is in addition to the existing coalescing of empty sectors.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

djs55/ocaml-vhd,xapi-project/ocaml-vhd