Retreive HKVM version from /etc/hkvm_version

Instead of /etc/isimage. Choice as been made to not enforce version
here as there are no needs to do so.

cloudcontrol/node/host/tags.py

@@ -213,9 +213,8 @@ def plugins(handler):
 
 # HKVM related tags
 def hkvmver():
-    """HKVM version."""
-    fields = dict(x.split(': ', 1) for x in open('/etc/isimage').read().split('\n') if x.strip())
-    return int(fields['image version'])
+    """HKVM version and patch level."""
+    return open('/etc/hkvm_version').read().strip()
 
 
 def hkvmsetup():