Fix import_path circular dependency by splitting it out.

Use import thread_pool instead from, from is evil.

Fix poweroff.out path for ./trace-boot.

run

@@ -309,6 +309,8 @@ Extra options to append at the end of the emulator command line.
         )
 
     def timed_main(self):
+        if self.env['emulator'] == 'native' and self.env['userland'] is None:
+            raise Exception('native emulator only supported in user mode')
         show_stdout = self.env['show_stdout']
         # Common qemu / gem5 logic.
         # nokaslr: