Just install these packages and restart the VM:
1.) Enable guest-utils on the host side:
sudo apt install virtualbox-guest-utils virtualbox-guest-x11
2.) Enable guest editions on the VMside
This must be done for each VM that you want to have the guest additons for accelerated GPU performance and for drag and drop/clipboard sharing
First insert the Guest Addtions CD image........
It seems that QEMU/KVM's default PS2 mouse and keyboard doesn't work right in most cases. I have especially observed issues using Ctrl+C and Ctrl+V and in Linux you may see repeated key presses in the terminal and you will wonder why you copied something and it's not in the clipboard when you try to paste. The way to temporarily fix it is to press the key that is repeating once(works in Linux but not really in Windows).
Sometimes when moving your mouse it will also s........