dom0/qmemman: call do_balance after each domain list change notification (#246)

For unknown reason watch '@releaseDomain' is called twice: first when domain
disappeared from xenstore, second when resources (including memory) are freed.
So call do_balance after each of this event to redistribute freed memory.

dom0/qmemman/qmemman.py

@@ -29,7 +29,6 @@ class SystemState:
 
     def del_domain(self, id):
         self.domdict.pop(id)
-        self.do_balance()
 
     def get_free_xen_memory(self):
         return self.xc.physinfo()['free_memory']*1024

dom0/qmemman/qmemman_server.py

@@ -29,11 +29,11 @@ class WatchType:
 class XS_Watcher:
     def __init__(self):
         self.handle = xen.lowlevel.xs.xs()
-        self.handle.watch('/local/domain', WatchType(XS_Watcher.domain_list_changed, None))
+        self.handle.watch('@introduceDomain', WatchType(XS_Watcher.domain_list_changed, None))
+        self.handle.watch('@releaseDomain', WatchType(XS_Watcher.domain_list_changed, None))
         self.watch_token_dict = {}
 
     def domain_list_changed(self, param):
-        time.sleep(0.05)
         curr = self.handle.ls('', '/local/domain')
         if curr == None:
             return
@@ -50,6 +50,7 @@ class XS_Watcher:
             self.watch_token_dict.pop(i)
             system_state.del_domain(i)
         global_lock.release()
+        system_state.do_balance()
 
     def meminfo_changed(self, domain_id):
         untrusted_meminfo_key = self.handle.read('', get_domain_meminfo_key(domain_id))