Debian's libvirt-daemon-system doesn't auto-create a default storage
pool (network exists but inactive) - added explicit pool-define/build/
start and net-start steps. Also fixed --cloud-init user-data=~/... not
expanding (tilde only expands at start of a shell word, not after =),
and noted why --os-variant may need to be an older release than the
actual image if osinfo-db is stale.
Disk images under k8s's home directory aren't readable by the
libvirt-qemu user that actually runs VMs under qemu:///system. Use
libvirt's own default storage pool (vol-create-as/vol-upload) instead,
so libvirtd handles file ownership rather than k8s directly, and pin
LIBVIRT_DEFAULT_URI=qemu:///system throughout.
