block/mirror: fix and improve do_sync_target_write

Use bdrv_dirty_bitmap_next_dirty_area() instead of
bdrv_dirty_iter_next_area(), because of the following problems of
bdrv_dirty_iter_next_area():

1. Using HBitmap iterators we should carefully handle unaligned offset,
as first call to hbitmap_iter_next() may return a value less than
original offset (actually, it will be original offset rounded down to
bitmap granularity). This handling is not done in
do_sync_target_write().

2. bdrv_dirty_iter_next_area() handles unaligned max_offset
incorrectly:

look at the code:
    if (max_offset == iter->bitmap->size) {
        /* If max_offset points to the image end, round it up by the
         * bitmap granularity */
        gran_max_offset = ROUND_UP(max_offset, granularity);
    } else {
        gran_max_offset = max_offset;
    }

    ret = hbitmap_iter_next(&iter->hbi, false);
    if (ret < 0 || ret + granularity > gran_max_offset) {
        return false;
    }

and assume that max_offset != iter->bitmap->size but still unaligned.
if 0 < ret < max_offset we found dirty area, but the function can
return false in this case (if ret + granularity > max_offset).

3. bdrv_dirty_iter_next_area() uses inefficient loop to find the end of
the dirty area. Let's use more efficient hbitmap_next_zero instead
(bdrv_dirty_bitmap_next_dirty_area() do so)

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>