With qemu versions prior to qemu-5.0 we'll format 'scsi=off' for
virtio-blk disks, but also for vhost-user-blk. This is a bug as it's not
supported.
Add a test case to show that wrong configuration is generated by adding
running 'disk-vhostuser' test case on capabilities from qemu-4.2.
For this to be possible it's required to enable shared memory via NUMA
configuration as old QEMU's don't allow configuration of the default
memory backend. This is achieved by adding a copy of the
'disk-vhostuser' XML with NUMA enabled.
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
