Unify the “--hypervisor” (params only) option

This unifies the two uses of --hypervisor-parameters with only options
(a=b,…) form. The patch changes a little SetInstanceParms since that
expected opts.hypervisor instead of opts.hvparams.

Note that there is another form of --hypervisor-parameters which also
takes the name, that comes in a separate patch.

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