make 'mgmt-permission:' event compatible with check-events script

It doesn't support str.format().

qubes/api/__init__.py

@@ -168,7 +168,7 @@ class AbstractQubesAPI(object):
 
     def fire_event_for_permission(self, **kwargs):
         '''Fire an event on the source qube to check for permission'''
-        return self.src.fire_event_pre('mgmt-permission:{}'.format(self.method),
+        return self.src.fire_event_pre('mgmt-permission:' + self.method,
             dest=self.dest, arg=self.arg, **kwargs)
 
     def fire_event_for_filter(self, iterable, **kwargs):