virt plugin: make indentation explicit

virt plugin: make indentation explicit

Fixes this warning raised by gcc-6's new "misleading-indentation"
option:

```
virt.c: In function 'lv_init':
virt.c:331:2: error: statement is indented as if it were guarded by... [-Werror=misleading-indentation]
  return 0;
  ^~~~~~

virt.c:328:5: note: ...this 'if' clause, but it is not
     if (virInitialize () != 0)
     ^~
```