virttest.standalone_test: Improve tmp files cleanup code

Make the tmp files cleanup code to run at the beginning
of the test suite execution, and also at the end. The
cleanup code was generalized and cleaned up, using the
new utility functions in aexpect and utils_net.

With this, at the end of a clean virt-test execution,
no tmp files are left behind.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
run: Kill vms and clean tmp files on Ctrl+C handler

Make the cleanup_env method more generic, and add it
to the KeyboardInterrupt handler of virt-test. This
way, users that do kill their virt-test execution
will have all vms and temporary files properly
cleaned.

This way, no temporary files will be around even
on Ctrl + C termination.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.data_dir: Add clean_tmp_files() utility function

Use it in the cleanup environment function as well.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>