Support was added to only add a WWN (World Wide Name) for a disk to libvirt via XML.
Cannot use virsh to define a WWN, rather the user must virsh edit the guest XML file and add the XML for the disk.
Add --wwn to virsh attach-disk command
The following command would attach the test.img to a virtual machine as a SCSI device via virsh
virsh attach-disk rhel7.1 /var/lib/libvirt/images/test.img sdb --driver qemu --subdriver raw --type disk --sourcetype file --wwn 5000c60016ea71ad --cache none