system: vm: Non-volatile 'run-vm.sh' creates a CoW image.

Previously, copying the image would consume a lot of space and was
I/O-intensive, to the point that the marionette connection timeout of
20s could be reached when running tests like "docker-system".

* gnu/system/vm.scm (common-qemu-options): Pass 'format=' for each
'-drive' option.
(system-qemu-image/shared-store-script)[copy-image]: New variable.
[builder]: Use it when VOLATILE? is false.

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