VirtIo errors inside VM with UFS-in-zvol

[Stephen Stuart]

I have VMs running on FreeBSD 10.0, for which I made zvols to provide the
block IO device for bhyve. When I boot from the installation ISO, gpart
works inside the VM, but newfs does not, reporting vtbd0 errors. Likewise,
dump does not work inside the VM (failing with too many vtbd0 hard errors),
but if I shut down the VM and dump the filesystem by specifying the
partition name in the host machine's ZFS space (/dev/zvol/zroot/VOLNAMEp2),
dump works just fine.

With a different VM, where I got smart and did all the gpart and newfs work
outside the VM:

    zfs create -V 10g zroot/WORKING
    gpart create -s GPT /dev/zvol/zroot/WORKING
    gpart add -t freebsd-boot -s 64k /dev/zvol/zroot/WORKING
    gpart add -t freebsd-ufs -s 7525m /dev/zvol/zroot/WORKING
    gpart add -t freebsd-swap /dev/zvol/zroot/WORKING
    newfs /dev/zvol/zroot/WORKINGp2

then install works fine inside the VM when I mount the filesystem and
create fstab by hand, and dump inside the VM works fine.

There's a somewhat related post here:

https://groups.google.com/forum/#!msg/mailing.freebsd.fs/Iw7aONDm-3Y/gecBCQls8oYJ

but I don't see anything specific to this issue. I'd like dump from inside
the VM to work, the fact that it doesn't makes me concerned that there are
issues lurking that might lead to data loss. Where do I go from here?

Thanks,
Stephen