virtio: postpone the execution of event_notifier_cleanup function

Use the EventNotifier's cleanup callback function to execute the
event_notifier_cleanup function after kvm unregistered the eventfd.

This change supports running the virtio_bus_set_host_notifier
function inside a memory region transaction. Otherwise, a closed
fd is sent to kvm, which results in a failure.

Signed-off-by: Gal Hammer <ghammer@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>