Extend virt-exportserver to handle export of qcow2

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

As a user I have to export raw disk image (gz) from the Export API, decompress it, and convert it to a qcow2 before pushing it to the container registry.

Describe the solution you'd like:

I want simply to export qcow2 disk image from the Export API, and push it to the container registry.

Describe alternatives you've considered:

N/A

Additional context:

There is no supported tooling for this, currently qemu-img lacks to stream the ouput of qcow2 while converting it from raw disk image.