ACRN didn't support dynamic memory allocation. SO it would reserve
a big page pool and use the GPA as index to get a page to do EPT
mapping. In order to save memory, we put high MMIO windows to [4G, 5G].
AFter we support dynamic page allocation for EPT mapping, we move
high MMIO windows to where it used to ([256G, 512G]) for 39 bits physical
address), we could move high memory to where is used to too ([4G, 4G +
size]).
Tracked-On: #5913
Signed-off-by: Tao Yuhong <yuhong.tao@intel.com>