streaming: switch to whole sectors in the interface rather than byte offsets

Everything is guaranteed to be sector-aligned, and this avoids having to
be careful to handle arbitrary byte ranges.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
test: verify the correct sectors are written in the streaming interface

We stream the disk contents and check that only the sectors we have
written are actually written, and all others are blank.

This test currently fails.

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