block/backup: unify different modes code path

block/backup: unify different modes code path

Do full, top and incremental mode copying all in one place. This
unifies the code path and helps further improvements.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190429090842.57910-5-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit c334e897d08eea1f5a3a95f6a2208afe6757c103)
*prereq for 110571be4e
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>