nspawn,machined: change default container image location from /var/lib/container...

nspawn,machined: change default container image location from /var/lib/container to /var/lib/machines

Given that this is also the place to store raw disk images which are
very much bootable with qemu/kvm it sounds like a misnomer to call the
directory "container". Hence, let's change this sooner rather than
later, and use the generic name, in particular since we otherwise try to
use the generic "machine" preferably over the more specific "container"
or "vm".