Re: kern/59618: occasional virtio block device lock ups/hangs

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

The attached patch adds a new paravirt_membar_sync() and teaches both
virtio(4) and hyperv(4) to use it.  ena(4) and any similar drivers
likely need the same treatment -- TBD.

(Xen will still use xen_mb() for now since it's also used in some
upstream Xen public header files.)

Giant diff:
https://www.NetBSD.org/~riastradh/tmp/20250831/pr59618-paravirtmbsync.diff

Itemized patch series:
https://www.NetBSD.org/~riastradh/tmp/20250831/pr59618-paravirtmbsync.patch

Review and testing welcome!