Hi,
VirtIO SCSI is a virtualized HBA that offers a couple of advantages
over the existing VirtIO block disk devices. Instead of having a vtbdX
disk, familiar daX and cdX devices are created like from other SCSI
drivers. A single HBA is able to support multiple devices unlike the
VirtIO block where there is one disk per PCI device. And SCSI devices
from the host are able to be passthrough to the guest.
VirtIO SCSI is relatively new - I think the Linux guest driver first
came out as part of the 3.4 kernel from last month - and as a result
you need a relatively new QEMU (post-February of this year).
Various minor bits of the spec do not seem to be supported by QEMU yet
(like event monitoring), but there is stub code in the driver for when
support is added.
Patch is available at:
www.daemoninthecloset.org/~bryanv/patches/freebsd/freebsd_virtio_scsi_1.patch
Should apply cleanly to any recent -current, and probably to a -stable
with minimal work.