virttest.defaults: Use autotest new distro API

Now that we have autotest 0.16 in EPEL, we can safely
use autotest.client.shared.distro. This means no more
horrible hacks have to be used to determine the appropriate
guest OS to use.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1640 from autotest/use-newer-autotest-api

virttest.defaults: Use autotest new distro API