vhost: correctly pass error to caller in vhost_dev_enable_notifiers()

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 b0b3db79559e57db340b292621c397e7a6cdbdc5 ("vhost-net: cleanup
host notifiers at last step")

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