virttest: Restore the working dir in download_test_provider

After check the repo informations we should restore the working
dir to original one. Otherwise it will cause problems after we
call it in our scripts.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1700 from ypu/asset

virttest: Restore the working dir in download_test_provider