block: don't lose data from last incomplete sector To read the last sector that is not aligned to sector boundary, current code for growable backends, since commit 893a8f6 "block: Produce zeros when protocols reading beyond end of file", drops the data and directly returns zeroes. That is incorrect. Signed-off-by: Fam Zheng <famz@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>