block/backup: move to copy_bitmap with granularity

block/backup: move to copy_bitmap with granularity

We are going to share this bitmap between backup and backup-top filter
driver, so let's share something more meaningful. It also simplifies
some calculations.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190429090842.57910-3-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit a8389e315ef71913ae99cf8f3b1f89e84631f599)
*prereq for 4a5b91ca
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>