Only allow VM snapshots to be taken on 'qcow2' disks

VM snapshot support in libvirt may not work correctly with some image
types, like 'raw'.

To make sure snapshots work correctly, only allow them to be taken on
VMs with 'qcow2' images, which is the most supported image type for that
operation.

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>
Revert "snapshot: Clone snapshots when cloning a VM"

This reverts commit c087f65f12af7394230c300ca63e3fd79fe65498.