block/nfs: add knob to set readahead

upcoming libnfs will feature internal readahead support.
Add a knob to pass the optional readahead value as a URL
parameter.

Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/nfs.c

@@ -321,6 +321,10 @@ static int64_t nfs_client_open(NFSClient *client, const char *filename,
             nfs_set_gid(client->context, val);
         } else if (!strcmp(qp->p[i].name, "tcp-syncnt")) {
             nfs_set_tcp_syncnt(client->context, val);
+#ifdef LIBNFS_FEATURE_READAHEAD
+        } else if (!strcmp(qp->p[i].name, "readahead")) {
+            nfs_set_readahead(client->context, val);
+#endif
         } else {
             error_setg(errp, "Unknown NFS parameter name: %s",
                        qp->p[i].name);