[varnish] bugfix, fix arg in _parse_varnishstat

Another change from #1213 introduced a regression by passing the
path of the command instead of the output of the comand.

checks.d/varnish.py

@@ -92,7 +92,7 @@ def check(self, instance):
         output, error = proc.communicate()
         if error and len(error) > 0:
             self.log.error(error)
-        self._parse_varnishstat(varnishstat_path, use_xml, tags)
+        self._parse_varnishstat(output, use_xml, tags)
 
         # Parse service checks from varnishadm.
         varnishadm_path = instance.get('varnishadm')