Document a workaround for an issue when running Tails with virt-manager

When booting images built from feature/bookworm with my usual libvirt configuration, tails-gdm-session fails to start, resulting boot to be stuck on a black screen with the cursor blinking.

Here are some relevant error messages from the journal:

gdm-shell-tails.desktop: LLVM ERROR: 64-bit code requested on a subtarget that doesn't support it! gdm-shell-tails.desktop: (EE) could not connect to wayland server gnome-session-binary: Unrecoverable failure in required component gdm-shell-tails.desktop

A workaround I found is to enable OpenGL in the Spice display settings. We decided to document that in the Graphics issues section of troubleshooting on the page for running Tails with virt-manager.