migration: ensure htab_save_first completes after timeout

htab_save_first_pass could return without finishing its work due to
timeout. The patch checks if another invocation of it is necessary and
will call it in htab_save_complete if necessary.

Signed-off-by: Jianjun Duan <duanj@linux.vnet.ibm.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
[removed overlong line]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
