Switch to python3 style 'except X as Y' notation

Which also works with python2.7

virtconv/ovf.py

@@ -370,7 +370,7 @@ class ovf_parser(parser_class):
 
         try:
             return _xml_parse_wrapper(xml, parse_cb)
-        except Exception, e:
+        except Exception as e:
             logging.debug("Error parsing OVF XML: %s", str(e))
 
         return False

virtconv/vmx.py

@@ -83,7 +83,7 @@ class _VMXFile(object):
             try:
                 lineobj = _VMXLine(line)
                 self.lines.append(lineobj)
-            except Exception, e:
+            except Exception as e:
                 raise Exception(_("Syntax error at line %d: %s\n%s") %
                     (len(self.lines) + 1, line.strip(), e))