[7.2.0] Qemu process hang with `defunct` when using `-blockdev` json property which file doesn't exists
Description of problem:
When using `throttle` and `throttle-group` to apply block device QOS,  
there is something wrong with check file exists validation.  
In upper commands, if the file which located `/mnt/b3b8dfb5-0a7c-4285-81d8-2bf8d33a3297/32c55f5a-96d1-4af4-a149-c95fd6652e3e/b016af76-f6b1-4614-b29a-78917924e55e` doesn't exist, it just hang with `defunct` process.  
![defunct](/uploads/607c81f0c5a490a50cd0882139fded4b/defunct.png)
Steps to reproduce:
1. Start Guest with upper command.
2. Hanged with defunct process
3.
Additional information:
![1682511478](/uploads/4c6ec5d32e71bf5c116ed146a27dc8a6/1682511478.png)  
- With GDB stack, i can find `no such file` error, but process don't exit