Update version for v4.1.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 26cc861..bcdac43 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-4.0.90
+4.0.91