MFC: r347960: bhyve virtio needs barriers

Under certain tight race conditions, we found that the lack of a memory
barrier in bhyve's virtio handling causes it to miss a NO_NOTIFY state
transition on block devices, resulting in guest stall. The investigation
is recorded in OS-7613. As part of the examination into bhyve's use of
barriers, one other section was found to be problematic, but only on
non-x86 ISAs with less strict memory ordering. That was addressed in
this patch as well, although it was not at all a problem on x86.

PR: 231117
Submitted by: Patrick Mooney <patrick.mooney@joyent.com>
Reviewed by: jhb, kib, rgrimes
Approved by: re (gjb), jhb
Differential Revision: https://reviews.freebsd.org/D19501