block-migration: fix blk_mig_save_dirty_block() return value checking

Commit 43be3a25c931a7f61a76fbfc9d35584cbfc5fb58 changed the
blk_mig_save_dirty_block() return code handling.  The function's doc
comment says:

  /* return value:
   * 0: too much data for max_downtime
   * 1: few enough data for max_downtime
   */

Because of the 1 return value, callers must check for ret < 0 instead of
just:

  if (ret) { ... }

We do not want to bail when 1 is returned, only on error.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1360534366-26723-3-git-send-email-stefanha@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
