virtio: export vring_notify as virtio_should_notify

virtio: export vring_notify as virtio_should_notify

Virtio dataplane needs to trigger the irq manually through the
guest notifier.  Export virtio_should_notify so that it can be
used around event_notifier_set.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>