Instead of hardcoded Xen commands. This will make it work for all
hypervisors, instead of duplicating hypervisor functionality in QA
itself.

The timeout has been removed as gnt-instance stop itself will make
sure the instance is down before returning. We just double-check that
it is indeed down.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>