xen: use mapcache for cpu_physical_memory_map

xen: use mapcache for cpu_physical_memory_map

this patch improves qemu-xen's mapcache to be able to guarantee that
certain mappings are going to last until explicitly unmapped.
This capability is necessary to implement cpu_physical_memory_map and
cpu_physical_memory_unmap that are part of the new dma api.
This patch also provides the implementation of these two functions,
removing the one based on bounce buffers that we are currently using.

Current DMA throughput: ~10MB/s
DMA throughput with this patch:  ~27MB/s

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>