The vhost-scsi-ccw backend is of type VHostSCSICcw, not VirtIOSCSICcw.

This fixes a segfault when invoking

    qemu-system-s390x -device vhost-scsi-ccw,?
Reviewed-by: NThomas Huth <thuth@linux.vnet.ibm.com>
Tested-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>