Move qemu outside of quay repo to its github repo

The quay-builder-qemu container image is built from qemu-coreos directory alone.
For cpaas integration, the case where two container images are built from
the same source repo is not directly supported.

To tackle this, the qemu-core-os directory would live in its own github repo : https://github.com/quay/quay-builder-qemu so that cpaas midstream repo for quay-builder-qemu will reference the new repo as its source (streamlining cpaas integration)