migration/block: fix pending() return value

Because of wrong return value of .save_live_pending() in
migration/block.c, migration finishes before the whole disk is
transferred. Such situation occurs when the migration process is fast
enough, for example when source and dest are on the same host.

If in the bulk phase we return something < max_size, we will skip
transferring the tail of the device. Currently we have "set pending to
BLOCK_SIZE if it is zero" for bulk phase, but there no guarantee, that
it will be < max_size.

True approach is to return, for example, max_size+1 when we are in the
bulk phase.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@parallels.com>
Message-id: 1419933856-4018-2-git-send-email-vsementsov@parallels.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 04636dc4)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>