KVM: selftests: Drop the "feature event" param from guest test helpers

Now that validation of event count is tied to hardware support for event,
and not to guest support for an event, drop the unused "event" parameter
from the various helpers.

No functional change intended.

Link: https://lore.kernel.org/r/20250117234204.2600624-5-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>