bhyve: Drop volatile qualifiers from virtio rings

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.

MFC after: 2 weeks
Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D37291