In the windows 8, the support for the XDDM (old windows display driver format) was removed. The format was replaced by WDDM and the “official” drivers won’t probably install.
In order to have a higher resolutions than 1024×768 on Windows 8 (or windows 10) guest:
– set the scrreen type to SPICE
– set QXL as the video card
– install drivers from: http://people.redhat.com/~vrozenfe/qxlwddm/qxlwddm-0.6.zip