Support for using existing qcow2 as a base image for qemu builds

[sachinphogat]

Is your feature request related to a problem? Please describe.

Feature to support existing qcow2 as base image for new qemu builds

Describe the solution you'd like

As part of qemu packer json, addition of one more builder to support existing qcow2 disk path.

Describe alternatives you've considered

Additional context

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/kind feature

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/lifecycle stale