vfio: add edid api for display (vgpu) devices.

This allows to set EDID monitor information for the vgpu display, for a
more flexible display configuration, using a special vfio region.  Check
the comment describing struct vfio_region_gfx_edid for more details.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>