The VirtIO standard supports two schemes for notification suppression: a notification enable bit and
a more sophisticated one (event_idx) that also supports delayed notifications.
Currently bhyve fully supports only the first scheme. This patch hides the notification
suppression internals by means of two inline routines, vq_kick_enable() and vq_kick_disable(),
and makes the code more readable.
Moreover, further improve readability by replacing the call to mb() with a call to
atomic_thread_fence_seq_cst(), which is already used in virtio.c