swiotlb-xen: make instances match their method names

Just drop two pointless _attrs prefixes to make the code a little
more grep-able.

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>