Add Virtual Disks to KVM

qemu-img create -f qcow2 /path/to/your/image.qcow2 40G
 

• before you attach the disk you have to start the virtual machine

virsh list -all
virsh start vm1

virsh attach-disk <vm-name> /path/to/your/image.qcow2 vdb --driver qemu --subdriver qcow2 --persistent