- support for libnfs API v2

Index: block/nfs.c
--- block/nfs.c.orig
+++ block/nfs.c
@@ -271,8 +271,13 @@ static int coroutine_fn nfs_co_preadv(BlockDriverState
     task.iov = iov;
 
     WITH_QEMU_LOCK_GUARD(&client->mutex) {
+#ifdef LIBNFS_API_V2
         if (nfs_pread_async(client->context, client->fh,
+                            NULL, bytes, offset, nfs_co_generic_cb, &task) != 0) {
+#else
+        if (nfs_pread_async(client->context, client->fh,
                             offset, bytes, nfs_co_generic_cb, &task) != 0) {
+#endif
             return -ENOMEM;
         }
 
@@ -317,9 +322,15 @@ static int coroutine_fn nfs_co_pwritev(BlockDriverStat
     }
 
     WITH_QEMU_LOCK_GUARD(&client->mutex) {
+#ifdef LIBNFS_API_V2
         if (nfs_pwrite_async(client->context, client->fh,
+                             buf, bytes, offset,
+                             nfs_co_generic_cb, &task) != 0) {
+#else
+        if (nfs_pwrite_async(client->context, client->fh,
                              offset, bytes, buf,
                              nfs_co_generic_cb, &task) != 0) {
+#endif
             if (my_buffer) {
                 g_free(buf);
             }
