On the KVM host:
root@kvm-host:~$ qemu-img convert -f qcow2 -O vmdk <path to source qcow2 image> <path to destination vmdk image>
On the VMware ESXi host:
root@esx-host:~$ vmkfstools -i <path to source vmdk image> <path to destination vmdk image> -d thin
With QEMU version 2.1 an later this can be reduced to a single command step:
root@kvm-host:~$ qemu-img convert -f qcow2 -O vmdk -o adapter_type=lsilogic,subformat=streamOptimized,compat6 <path to source qcow2 image> <path to destination vmdk image>