qemu-img: rebase: Reuse in-chain BlockDriverState

If a chain was detected, don't open a new BlockBackend from the target
backing file which will create a new BlockDriverState. Instead, create
an empty BlockBackend and attach the already open BlockDriverState.

Permissions for blk_new() were copied from blk_new_open() when
flags = 0.

Reviewed-by: Karl Heubaum <karl.heubaum@oracle.com>
Reviewed-by: Eyal Moscovici <eyal.moscovici@oracle.com>
Signed-off-by: Sagi Amit <sagi.amit@oracle.com>
Co-developed-by: Sagi Amit <sagi.amit@oracle.com>
Signed-off-by: Sam Eiderman <shmuel.eiderman@oracle.com>
Message-id: 20190523163337.4497-4-shmuel.eiderman@oracle.com
Signed-off-by: Max Reitz <mreitz@redhat.com>