Make qfilexchgd listen for change in /vm to detect vm start/stop

... instead of watching /local/domain, which changes whenever meminfo-wwriter
pushes data.

dom0/pendrive_swapper/qfilexchgd

@@ -276,7 +276,7 @@ class DomainState:
 class XS_Watcher:
     def __init__(self):
         self.handle = xen.lowlevel.xs.xs()
-        self.handle.watch('/local/domain', WatchType(XS_Watcher.dom_list_change, None))
+        self.handle.watch('/vm', WatchType(XS_Watcher.dom_list_change, None))
         self.domdict = {}
 
     def dom_list_change(self, param):