qubes/vm: do not recursively use repr in BaseVM.__repr__

This will easily end up in infinite recursion. For example
'sys-net'.template points at 'fedora-23', which itself has
'fedora-23'.netvm set to 'sys-net'.

qubes/vm/__init__.py

@@ -167,7 +167,7 @@ def __repr__(self):
         proprepr = []
         for prop in self.property_list():
             try:
-                proprepr.append('{}={!r}'.format(
+                proprepr.append('{}={!s}'.format(
                     prop.__name__, getattr(self, prop.__name__)))
             except AttributeError:
                 continue