dom0: qmemman: set MIN_PREFMEM to 200MB

Apparently 100MB is not enough for fc15 template and it causes some VMs to be killed internally by OOM during their initialization.

dom0/qmemman/qmemman_algo.py

@@ -58,7 +58,7 @@ def refresh_meminfo_for_domain(domain, untrusted_xenstore_key):
                         
 def prefmem(domain):
     CACHE_FACTOR = 1.3
-    MIN_PREFMEM = 100*1024*1024
+    MIN_PREFMEM = 200*1024*1024
 #dom0 is special, as it must have large cache, for vbds. Thus, give it a special boost
     if domain.id == '0':
         return min(domain.mem_used*CACHE_FACTOR + 350*1024*1024, domain.memory_maximum)