Fix changing the list of enabled hypervisors

When enabling a new hypervisor, we must ensure that we have its
parameters in the cluster (global) hvparams dictionary.

Furthermore, we add a verify check for this case (this should be better
replaced with an auto-fix?).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>