iters: More debug on DUMP req/resp processing

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
iters: Properly handle failed dump

When processing DUMP resps we may encounter not NOTIFY
one, but the DUMP as well, meaning that dump failed.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
iters: Use link_remap option

For migration link remaps can be used.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>