migration: Fix possible infinite loop of ram save process

migration: Fix possible infinite loop of ram save process

When starting ram saving procedure (especially at the completion phase),
always set last_seen_block to non-NULL to make sure we can always correctly
detect the case where "we've migrated all the dirty pages".

Then we'll guarantee both last_seen_block and pss.block will be valid
always before the loop starts.

See the comment in the code for some details.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>