vhost: Always store new kick fd on vhost_svq_set_svq_kick_fd

vhost: Always store new kick fd on vhost_svq_set_svq_kick_fd

We can unbind twice a file descriptor if we call twice
vhost_svq_set_svq_kick_fd because of this. Since it comes from vhost and
not from SVQ, that file descriptor could be a different thing that
guest's vhost notifier.

Likewise, it can happens the same if a guest start and stop the device
multiple times.

Reported-by: Lei Yang <leiyang@redhat.com>
Fixes: dff4426fa6 ("vhost: Add Shadow VirtQueue kick forwarding capabilities")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>