[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>

tools/python/xen/xend/server/DevController.py

@@ -207,6 +207,9 @@ class DevController:
 
         devid = int(devid)
 
+        # Modify online status /before/ updating state (latter is watched by
+        # drivers, so this ordering avoids a race).
+        self.writeBackend(devid, 'online', "0")
         self.writeBackend(devid, 'state', str(xenbusState['Closing']))
 
 
@@ -406,7 +409,8 @@ class DevController:
             'domain' : self.vm.getName(),
             'frontend' : frontpath,
             'frontend-id' : "%i" % self.vm.getDomid(),
-            'state' : str(xenbusState['Initialising'])
+            'state' : str(xenbusState['Initialising']),
+            'online' : "1"
             })
 
         return (backpath, frontpath)