Minor fixes for bashate trunk

We don't have a new bashate release yet, but this fixes some minor
issues when used with bashate trunk.

The only two things triggered are stricter checking of assignment in
local declartions (catches quotes) and one instance of evaluating
arithmetic result in tools/xen/functions.

Therefore, hopefully, this change has no effect!

Change-Id: I3fd5d55236e90c2f2c25243af87adaf0c1b763e2
diff --git a/tox.ini b/tox.ini
index f9d04f2..bdfd29d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -8,6 +8,9 @@
 install_command = pip install {opts} {packages}
 
 [testenv:bashate]
+# if you want to test out some changes you have made to bashate
+# against devstack, just set BASHATE_INSTALL_PATH=/path/... to your
+# modified bashate tree
 deps =
    {env:BASHATE_INSTALL_PATH:bashate==0.3.2}
 whitelist_externals = bash