An iothread for virtio-scsi is a property of the controller. Add a lookup
of the 'virtio-scsi-pci' and 'virtio-scsi-ccw' device properties and parse
the output.  For both, support for the iothread was added in qemu 2.4
while support for virtio-scsi in general was added in qemu 1.4.

Modify the various mock capabilities replies (by hand) to reflect the
when virtio-scsi was supported and then specifically when the iothread
property was added. For versions prior to 1.4, use the no device error
return for virtio-scsi. For versions 1.4 to before 2.4, add some data
for virtio-scsi-pci even though it isn't complete we're not looking for
anything specific there anyway. For 2.4 to 2.6, add a more complete reply.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>