tests/gui-vm: Expose more options as cli flags

Previously we didn't have any launcher with display support which could boot from virtio-blk. This is quite useful for testing the GPU/display against various guests.

This is used by my (WIP) utility here https://github.com/mtjhrc/krun-fedora-gui-tester which setups and runs Fedora with stuff you might want for debugging GPU/display/virtio-input (preinstalled things like glmark, vkcube, weston, gnome, ssh already setup...)