qxl_pre_init: fix calculation of available video memory
qxl_pre_init: fix calculation of available video memory

Don't forget to substract monitors_config area.

Memory map of qxl RAM looks something like the following:
---------------------
|                   |
| Surface0 Area     |
|                   |
---------------------
|                   |
|                   |
| Video Pages       |
|                   |
|                   |
---------------------
| Monitors Config   |
---------------------
| RAM Header        |
---------------------

facboy/xserver-xorg-video-qxl,fgouget/xf86-video-qxl,facboy/xserver-xorg-video-qxl,fgouget/xf86-video-qxl,facboy/xserver-xorg-video-qxl,fgouget/xf86-video-qxl,facboy/xserver-xorg-video-qxl,fgouget/xf86-video-qxl