qxl_driver: introduce helpers for memory map and unmap
qxl_driver: introduce helpers for memory map and unmap

qxl_map_memory_helper and qxl_unmap_memory_helper introduced, here as
a simple split, later different implementation if XSPICE defined will
be added. Nicer then putting an ifdef inside the map/unmap functions.

facboy/xserver-xorg-video-qxl,sombrafam/virtioqxl-xf86-video-qxl,dezelin/xf86-video-qxl,fgouget/xf86-video-qxl,facboy/xserver-xorg-video-qxl,sombrafam/virtioqxl-xf86-video-qxl,fgouget/xf86-video-qxl,fgouget/xf86-video-qxl,dezelin/xf86-video-qxl,dezelin/xf86-video-qxl,sombrafam/virtioqxl-xf86-video-qxl,fgouget/xf86-video-qxl,facboy/xserver-xorg-video-qxl,facboy/xserver-xorg-video-qxl