gnu: vm: Set the right permissions and ownership on directories.

* gnu/system/vm.scm (qemu-image): Change the store's mode to #o1775.
  Support 'populate' clauses that specify a UID and GID.
  (system-qemu-image): Make sure /nix/store has owner 'root' and group
  'guixbuild'.  Set the right owner for /home/guest.  Create
  /var/nix/profiles/per-user/{root,guest}.