xen: use uint64_t instead of target_ulong in cpu_ioreq_move

cpu_ioreq_move might move 8 bytes at a time so we must make sure that
the temporary variable can hold 8 bytes.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>

xen-all.c

@@ -620,7 +620,7 @@ static void cpu_ioreq_move(ioreq_t *req)
             }
         }
     } else {
-        target_ulong tmp;
+        uint64_t tmp;
 
         if (req->dir == IOREQ_READ) {
             for (i = 0; i < req->count; i++) {