Update version for v5.1.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index e427b98..023d5c9 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-5.0.90
+5.0.91