Fixes bug 723235

The XML templates have been converted into properties, thus we can
compare the mtime of the XML templates (libvirt and cpuinfo) each
time they are needed, checking if they have been modified and
reloading them. Added a function to read cached files.

Change-Id: I6cf0229c6435300e73f9d9a6b10b0bf9bf144a55

nova/utils.py

@@ -1110,3 +1110,17 @@ def sanitize_hostname(hostname):
     hostname = hostname.strip('.-')
 
     return hostname
+
+
+def read_cached_file(filename, cache_info):
+    """Return the contents of a file. If the file hasn't changed since the
+    last invocation, a cached version will be returned.
+    """
+    mtime = os.path.getmtime(filename)
+    if cache_info and mtime == cache_info.get('mtime', None):
+        return cache_info['data']
+
+    data = open(filename).read()
+    cache_info['data'] = data
+    cache_info['mtime'] = mtime
+    return data