Install only linux-image-cloud-amd64 in VMs for >=buster and amd64

With linux-image-cloud-amd64 we only need ~96MB of disk space, whereas
the corresponding linux-image-amd64 takes about 317MB of disk space.
Also there's usually no need to install linux-headers and firmware
packages inside a VM, so skip those as well.

Closes: grml/grml-debootstrap#178