swiotlb-xen: implement xen_swiotlb_dma_mmap callback

This function creates userspace mapping for the DMA-coherent memory.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Oleksandr Dmytryshyn <oleksandr.dmytryshyn@globallogic.com>
Signed-off-by: Andrii Anisov <andrii_anisov@epam.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad@kernel.org>