migration: put the finish part into a new function

This patch only moved the last part of migration_thread() into a new
function migration_iteration_finish() to make it much shorter.  With
previous works to remove some local variables, now it's fairly easy to
do that.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>