QA: Verify hypervisors, make available through functions

Some parts of QA should only be run with certain hypervisors (e.g.
testing instance HV parameters). To make that happen we always define
the cluster's hypervisors and provide a function to get the default
hypervisor.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>