XenAPI: Fix declare definition

Some versions of bash do not initialise a variable with declare -a
therefore with set -u bash can terminate the script.

Be more verbose in declaring the array if it is not set

Change-Id: I6ec2b6e986aeffe539a2ab93432fa7af9e5a4f5d

tools/xen/scripts/on_exit.sh

@@ -3,7 +3,9 @@
 set -e
 set -o xtrace
 
-declare -a on_exit_hooks
+if [ -z "${on_exit_hooks:-}" ]; then
+    on_exit_hooks=()
+fi
 
 on_exit()
 {