xen-netback: remove 'hotplug-status' once it has served its purpose

Removing the 'hotplug-status' node in netback_remove() is wrong; the script
may not have completed. Only remove the node once the watch has fired and
has been unregistered.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>