don't install xf86-video-qxl to work around it segfaulting when used

Issue #235 (closed) describes an issue that startx segfaults on qemu/kvm virtual machine with qxl graphics device. Not installing the xf86-video-qxl driver works around this, because then the framebuffer driver is automatically used as fallback.

This is not optimal, because the fb driver is slower than qxl. But it is still very usable and should serve as solution until upstream develops a better fix.

Fixes #235 (closed)