Update version for v8.1.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 46a6526..cbe9ed9 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-8.0.90
+8.0.91