kvm: Clean up irqfd API

No need to expose the fd-based interface, everyone will already be fine
with the more handy EventNotifier variant. Rename the latter to clarify
that we are still talking about irqfds here.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
