kvm: remove last version-based feature detection

This was left behind because it required a different kvm invocation.
Now that we can add new ones cheaply (two constants) it's easy to get
rid of it. Differently than in other cases we support old version which
don't have the actual option, and error out when it's invoked.

This also removes two hardcoded strings, and brings them at constant
level. A lot more are present, but two is megl che one (cit).

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>