backup: Copy only dirty areas

backup: Copy only dirty areas

The backup job must only copy areas that the copy_bitmap reports as
dirty.  This is always the case when using traditional non-offloading
backup, because it copies each cluster separately.  When offloading the
copy operation, we sometimes copy more than one cluster at a time, but
we only check whether the first one is dirty.

Therefore, whenever copy offloading is possible, the backup job
currently produces wrong output when the guest writes to an area of
which an inner part has already been backed up, because that inner part
will be re-copied.

Fixes: 9ded4a0114968e98b41494fc035ba14f84cdf700
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190801173900.23851-2-mreitz@redhat.com
Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>