So one thing that has been on my mind for a while:  I'd really like
to kill the separate dma ops in Xen swiotlb.  If we compare xen-swiotlb
to swiotlb the main difference seems to be:

 - additional reasons to bounce I/O vs the plain DMA capable
 - the possibility to do a hypercall on arm/arm64
 - an extra translation layer before doing the phys_to_dma and vice
   versa
 - an special memory allocator

I wonder if inbetween a few jump labels or other no overhead enablement
options and possibly better use of the dma_range_map we could kill
off most of swiotlb-xen instead of maintaining all this code duplication?