virttest.utils_test: Make run_virt_sub_test work with new file structure

Update the test directories according to new file structure. Additionally
fix the variable names.

This patch doesn't change the functionality, only updates the structure. Anyway I'd like to __raise a question__, shouldn't we use the test names instead of the script name? Currently we have multiple test providers and the script names can collide. That's why I added the `logging.debug("Running virt subtest '%s'", module_path)` to log to be able to distinguish which actual test was used.

Kind regards,
Lukáš