migration: Fix postcopy bw for recovery

We've got max-postcopy-bandwidth parameter but it's not applied
correctly after a postcopy recovery so the recovered migration stream
will still eat the whole net bandwidth.  Fix that up.

Reported-by: Xiaohui Li <xiaohli@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20190906130103.20961-1-peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>