Update version for v2.10.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index a0700e6..161b595 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.9.90
+2.9.91