sparse_dd: avoid writing to the underlying vhd file
sparse_dd: avoid writing to the underlying vhd file

This isn't safe unless the tapdisk is paused, since it might
rewrite (and therefore corrupt) a footer on close.

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

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