Mini-OS: don't crash if no shutdown node is available

Mini-OS: don't crash if no shutdown node is available

It might be perfectly fine not to have a control/shutdown Xenstore
node. If this is the case, don't crash, but just terminate the
shutdown thread after issuing a message that shutdown isn't available.

In fini_shutdown() clearing the watch can result in an error now, in
case the early exit above was taken. Just ignore this error now.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>