Ubuntu Mantic minimized images contains lxd-agent-loader for all minimized image builds

A recent change to the cloud-minimal seed introduced `lxd-agent-loader` as a dependency of the cloud-minimal seed (see https://git.launchpad.net/~ubuntu-core-dev/ubuntu-seeds/+git/ubuntu/commit/?id=c962a46f3596c0eaeff1469ee4e9c46d3d23ebf4 ).

This seed is used by livecd-rootfs for ubuntu-cpc project minimized subproject image builds.

This means that `lxd-agent-loader` is installed in all minimized mantic images built under the ubuntu-cpc project - this extends past the public images and includes GCE, AWS, Azure, Oracle etc.

It has always been the case that `lxd-agent-loader` was present in these minimized images so it is not a regression but given the recent initiative to make Mantic minimized images smaller and faster, it is a perfect opportunity to make a change so that `lxd-agent-loader` is only present in the images that LXD actually uses as VM images - this is the download qcow2 images @ http://cloud-images.ubuntu.com/minimal/daily/mantic/

This bug is to track the work to only install `lxd-agent-loader` in the qcow image or in the images that the qcow2 image depends on and not any of the other cloud images.

This would mean a change in the https://git.launchpad.net/~ubuntu-core-dev/ubuntu-seeds/+git/ubuntu/tree/cloud-minimal seed too.