xen: Log errno rather than return value

xen_modified_memory() sets errno to communicate what went wrong so log
this rather than the return value which is not interesting.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>

hw/i386/xen/xen-hvm.c

@@ -1446,7 +1446,7 @@ void xen_hvm_modified_memory(ram_addr_t start, ram_addr_t length)
         if (rc) {
             fprintf(stderr,
                     "%s failed for "RAM_ADDR_FMT" ("RAM_ADDR_FMT"): %i, %s\n",
-                    __func__, start, nb_pages, rc, strerror(-rc));
+                    __func__, start, nb_pages, errno, strerror(errno));
         }
     }
 }