build-support/vm: Fix missing sleep command (#476230)

      ${pkgs.virtiofsd}/bin/virtiofsd --xattr --socket-path virtio-xchg.sock --sandbox none --seccomp none --shared-dir xchg &

      # Wait until virtiofsd has created these sockets to avoid race condition.

      until [[ -e virtio-store.sock ]]; do sleep 1; done

      until [[ -e virtio-xchg.sock ]]; do sleep 1; done

      until [[ -e virtio-store.sock ]]; do ${coreutils}/bin/sleep 1; done

      until [[ -e virtio-xchg.sock ]]; do ${coreutils}/bin/sleep 1; done

      ${qemuCommand}

      EOF