bhyve: Drop volatile qualifiers from virtio rings

[markj]

Summary

The qualifiers are there presumably because these rings are mapped into
the guest, but they do not appear to be required for correctness, and
bhyve generally doesn't qualify accesses to guest memory this way.
Moreover, the qualifiers are discarded by snapshot code, causing clang
to emit warnings. Just stop using volatile here.

[jhb]

I can see a reason for having volatile pointers for things shared with the guest, though I think instead the right approach is to use memory barriers in the device models when working with data structures shared with a guest.