We override the error value r in fail_vq, this will cause the caller
can't detect the failure which may cause the caller may disable the
notifiers twice if vhost is failed to start. Fix this by using another
variable to keep track the return value of set_host_notifier().

Fixes b0b3db79 ("vhost-net: cleanup
host notifiers at last step")

Cc: qemu-stable@nongnu.org
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>