RFE: Add new target for virt-install which, produces .qcow image; previous xz target will depend on the .qcow output, allowing qcow image to be provided instead of ISO

[vvaldez]

This will allow a qcow image to be provided instead of an ISO, allowing the virt-install target to be skipped if say, the rhel-guest-image-7-*.qcow2 cloud image is used instead of an ISO. This also splits apart the target so an ISO install over virt-install can be completed separately from the qcow file being compressed. This would also remove the requirement to give qemu access to the user's directory.