conf: Add <hostdev model='virtio-{non-}transitional'/>

conf: Add <hostdev model='virtio-{non-}transitional'/>

qemu vhost-scsi devices map to XML roughly like:

    <hostdev mode='subsystem' type='scsi_host'>
      <source protocol='vhost' wwpn=X/>
    </hostdev>

To support vhost-scsi-pci-{non-}traditional in qemu, we
need to to extend the SCSI Host hostdev XML to handle
model= value. This matches the XML model= format used
for mediated devices. This is just the domain_conf bits
and some XML test cases.

Use of virtio-X naming here does not match the hostdev
protocol=vhost nor does it match the qemu vhost-X device
naming, however it's more consistent with all other
model= names in this area, and also matches the
inconsistency of <vsock> devices which use model=virtio
but map to vhost-vsock on the qemu commandline

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>