Fix not working qemu-guest-agent in Proxmox running DietPi VM

fix-dietpi-proxmox-qemu-guest-agent.sh

# References
# - https://dietpi.com/forum/t/qemu-guest-agent-shutdown/5898/7
# - https://pve.proxmox.com/wiki/Qemu-guest-agent

# 1. Here are the commands to run
apt install qemu-guest-agent \         # install guest agent
&& systemctl unmask systemd-logind \
&& apt install dbus \
&& systemctl start systemd-logind \
&& systemctl status systemd-logind     # check status to verify

# 2. Poweroff the VM and g
# 3. Go to Options > QEMU Guest Agent > Enabled
# 4. Start VM and test Shutdown from Proxmox Webui