Произвожу снапшот только диска vda:
virsh snapshot-create-as --domain "s321" "s321-state-2018-10-11.144739" --no-metadata --atomic --disk-only --diskspec vda,file=/volume0/s321/s321-state-vda-2018-10-11.144739.gcow2
error: unsupported configuration: source for disk 'vdb' is not a regular file; refusing to generate external snapshot name
virsh domblklist s321 --details
Type Device Target Source
------------------------------------------------
file cdrom hda -
file cdrom hdb /home/virtio-win.iso
block disk vda /dev/vg1/s321
block disk vdb /dev/vg1/s321_2