Fixup hypervisor queries in node query

We need to only query the default (first enabled) hypervisor, not all
hypervisors. For this, we need to add a manual check to ensure that we
don't have a corrupt config (there's no "NonEmptyList" type…).

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