swiotlb-xen: use ->map_page to implement ->map_sg

We can simply loop over the segments and map them, removing lots of
duplicate code.

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>