xen, gfx passthrough: register a isa bridge Currently we just register this isa bridge when we use IGD passthrough in Xen side. Signed-off-by: Tiejun Chen <tiejun.chen@intel.com> Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com> Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>