migration: Sync requested pages after postcopy recovery

We synchronize the requested pages right after a postcopy recovery happens.
This helps to synchronize the prioritized pages on source so that the faulted
threads can be served faster.

Reported-by: Xiaohui Li <xiaohli@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20201021212721.440373-5-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>