block-migration: fix block_save_iterate() return value

The .save_live_iterate() function returns 0 to continue iterating or 1
to stop iterating.

Since 16310a3cca7320edb9341c976f7819de0a8c27e0 it only ever returns 0,
leading to an infinite loop.

Return 1 if we have finished sending dirty blocks.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>