block/backup: refactor: split out backup_calculate_cluster_size

Split out cluster_size calculation. Move copy-bitmap creation above
block-job creation, as we are going to share it with upcoming
backup-top filter, which also should be created before actual block job
creation.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190429090842.57910-6-vsementsov@virtuozzo.com
[mreitz: Dropped a paragraph from the commit message that was left over
         from a previous version]
Signed-off-by: Max Reitz <mreitz@redhat.com>