only apply tcg plugin in a certain process in system mode

[syheliel syheliel]

I want to use tcg plugin to trace a particular process in system mode. But I find that the current tcg plugin will work on all process:
```

./qemu-system-x86_64 -plugin tests/plugin/libinsn.so \
    -d plugin ../tests/tcg/multiarch/threadcount
```
above command will apply libinsn.so to all process and generate a large output. How can I get pid or sth else to help me only register the tcg plugin to a certain process?

- syheliel