xen, gfx passthrough: add opregion mapping

xen, gfx passthrough: add opregion mapping

The OpRegion shouldn't be mapped 1:1 because the address in the host
can't be used in the guest directly.

This patch traps read and write access to the opregion of the Intel
GPU config space (offset 0xfc).

The original patch is from Jean Guyader <[email protected]>

Signed-off-by: Tiejun Chen <[email protected]>
Signed-off-by: Yang Zhang <[email protected]>
Acked-by: Stefano Stabellini <[email protected]>
Signed-off-by: Stefano Stabellini <[email protected]>