Document KVM configuration for better graphic performance

We could add some more information to
https://tails.boum.org/doc/advanced_topics/virtualization/virt-manager/

The only KVM config in which I’ve seen acceptable graphics performance is QXL with lots of video memory (starting with Tails 2.4, as before it was hopeless).

Related issues

Related to #14786 (closed)

Original created by @emmapeel on 11555 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information