Use Virtualbox guest kernel modules for Arch Kernel (virtualbox-guest-modules-arch).

This commit fixes the 'file not found: `fsck.btrfs''-Error during
system upgrade.

added polkit to pacstrap for systemctl poweroff, added sudo to poweroff
command, moved link for systemd-resolved to postinstall.sh

added new postinstall.sh, moved the controllers to postinstall.sh and added the script to vagrant.json