Support backing image and destroy VMs before volumes

[JPvRiel]

• Should fix https://github.com/stackhpc/ansible-role-libvirt-vm/issues/18
• Backing images with COW can be a bit faster than full image clones
• Minor bash script variable quoting to silence shellcheck

[JPvRiel]

Build failures seem to be due to new syntax checks introduced since the last build and are not functionally related to the PR changes.

On my side, did quick sanity check creating and destroying VMs with image, backing_image or neither option set.

[markgoddard]

I've submitted a PR to fix the ansible-lint CI errors https://github.com/stackhpc/ansible-role-libvirt-vm/pull/20/files.