MFi386 r1.94: If XEN, make pmap_kextract = pmap_kextract_ma. This is a

no-op currently, since FreeBSD/amd64 doesn't have (paravirtualized) Xen
support, but if/when that support is ever added we'll want this, and
until then it's harmless.

sys/amd64/amd64/busdma_machdep.c

@@ -141,6 +141,11 @@ int run_filter(bus_dma_tag_t dmat, bus_addr_t paddr);
 int _bus_dmamap_count_pages(bus_dma_tag_t dmat, bus_dmamap_t map, pmap_t pmap,
     void *buf, bus_size_t buflen, int flags);
 
+#ifdef XEN
+#undef pmap_kextract
+#define pmap_kextract pmap_kextract_ma
+#endif
+
 /*
  * Return true if a match is made.
  *