bhyve: virtio: introduce vq_kick_enable() and vq_kick_disable()

bhyve: virtio: introduce vq_kick_enable() and vq_kick_disable()

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

Reviewed by:	pmooney_pfmooney.com, bryanv
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D20581