core: prevent permissions error when VM was started by root

When VM is started by root, config file is created with root owner and
user has no write access to it. As the directory is user-writable,
delete the file first.

Conflicts:
	core-modules/000QubesVm.py

core-modules/000QubesVm.py

@@ -1090,6 +1090,8 @@ class QubesVm(object):
         # FIXME: This is only for debugging purposes
         old_umask = os.umask(002)
         try:
+            if os.path.exists(file_path):
+                os.unlink(file_path)
             conf_appvm = open(file_path, "w")
             conf_appvm.write(domain_config)
             conf_appvm.close()