[XENBUS] Introduce new "online" node for backend drivers.

[XENBUS] Introduce new "online" node for backend drivers.

Driver changes:  Make backend drivers check it when entering Closed
state, only unregister devices when the "online" node is either "0" or
not present.  The later maintains backward compatibility.

Tools changes:  Update "online" node when creating and hot-unplugging
devices.

Background: When kexec'ing a new kernel, the old kernel's frontend
driver will shutdown, but the backend device should *NOT* disappear
due to that, so the new kernel instance can reconnect.

Signed-off-by: Gerd Hoffmann <kraxel@suse.de>