backup: simplify non-dirty bits progress processing

backup: simplify non-dirty bits progress processing

Set fake progress for non-dirty clusters in copy_bitmap initialization,
to. It simplifies code and allows further refactoring.

This patch changes user's view of backup progress, but formally it
doesn't changed: progress hops are just moved to the beginning.

Actually it's just a point of view: when do we actually skip clusters?
We can say in the very beginning, that we skip these clusters and do
not think about them later.

Of course, if go through disk sequentially, it's logical to say, that
we skip clusters between copied portions to the left and to the right
of them. But even now copying progress is not sequential because of
write notifiers. Future patches will introduce new backup architecture
which will do copying in several coroutines in parallel, so it will
make no sense to publish fake progress by parts in parallel with
other copying requests.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 20171012135313.227864-5-vsementsov@virtuozzo.com
Signed-off-by: Jeff Cody <jcody@redhat.com>