pool_xml is a multi-line string of contents here, not a file's path or in one line.

So it caused this error:
```
TestFail: /bin/bash: -c: line 0: syntax error near unexpected token `newline'
16:15:10 ERROR| /bin/bash: -c: line 0: `/usr/bin/virsh pool-define --file <pool type='dir'>'
```