sparse_dd: when bypassing the tapdisk for writing, assume destination is not prezeroed
sparse_dd: when bypassing the tapdisk for writing, assume destination is not prezeroed

The destination will be full of the old vhd metadata (and possibly
arbitrary other data)

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

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