Catch crash error in IOU in case of permission denied

Fix #186

gns3server/modules/iou/iou_vm.py

@@ -433,7 +433,10 @@ class IOUVM(BaseVM):
 
             yield from self._library_check()
 
-            self._rename_nvram_file()
+            try:
+                self._rename_nvram_file()
+            except OSError as e:
+                raise IOUError("Could not rename nvram files: {}".format(e))
 
             iourc_path = self.iourc_path
             if iourc_path is None: