drm/i915/gvt: throw error on unhandled vfio ioctls

On unknown/unhandled ioctls the driver should return an error, so
userspace knows it tried to use something unsupported.

Cc: stable@vger.kernel.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>