block: Support streaming to an intermediate layer

block: Support streaming to an intermediate layer

This makes sure that the image we are streaming into is open in
read-write mode during the operation.

Operation blockers are also set in all intermediate nodes, since they
will be removed from the chain afterwards.

Finally, this also unblocks the stream operation in backing files.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>