block/backup: simplify backup_incremental_init_copy_bitmap

block/backup: simplify backup_incremental_init_copy_bitmap

Simplify backup_incremental_init_copy_bitmap using the function
bdrv_dirty_bitmap_next_dirty_area.

Note: move to job->len instead of bitmap size: it should not matter but
less code.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190429090842.57910-2-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit c2da3413c021398152e98022261bb1643276a2fe)
*prereq for 4a5b91ca
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>