Also update Kconfig help text, explaining things:

Cirrus is obsolete, the hardware was designed in the 90ies
and can't keep up with todays needs.  More background:
https://www.kraxel.org/blog/2014/10/qemu-using-cirrus-considered-harmful/

Better alternatives are:
  - stdvga (DRM_BOCHS, qemu -vga std, default in qemu 2.2+)
  - qxl (DRM_QXL, qemu -vga qxl, works best with spice)
  - virtio (VIRTIO_GPU), qemu -vga virtio)
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>