dom0: qmemman: distribute memory only if there are VMs which can accept it

This prevent potential inifinite loop in qmemman when free memory cannot be
assigned to any VM (because of static max). Practically this will never happen,
because dom0 can always accept memory.

dom0/qmemman/qmemman_algo.py

@@ -126,7 +126,7 @@ def balance_when_enough_memory(domain_dictionary, xen_free_memory, total_mem_pre
             acceptors_count += 1
         target_memory[i] = target
 # distribute left memory across all acceptors
-    while left_memory > 0:
+    while left_memory > 0 and acceptors_count > 0:
         print '  left_memory:', left_memory, 'acceptors_count:', acceptors_count
         new_left_memory = 0
         for i in target_memory.keys():