swiotlb-xen: implement xen_swiotlb_get_sgtable callback

Signed-off-by: Andrii Anisov <andrii_anisov@epam.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad@kernel.org>