XenAPI: Fix new useage of trueorfalse

* Ensure that Xen setup scripts will continue to function when unset
  variables are used in stackrc
* Ensure that the generic functions are sourced in all places that xenrc
  (which sources stackrc) is sourced.

Change-Id: I54eba20733c2e149621b74a1387f0bef14fca12e
diff --git a/tools/xen/build_xva.sh b/tools/xen/build_xva.sh
index 958102b..fbbfd6f 100755
--- a/tools/xen/build_xva.sh
+++ b/tools/xen/build_xva.sh
@@ -21,9 +21,19 @@
 # This directory
 TOP_DIR=$(cd $(dirname "$0") && pwd)
 
+# Source lower level functions
+. $TOP_DIR/../../functions
+
 # Include onexit commands
 . $TOP_DIR/scripts/on_exit.sh
 
+# xapi functions
+. $TOP_DIR/functions
+
+# Determine what system we are running on.
+# Might not be XenServer if we're using xenserver-core
+GetDistro
+
 # Source params - override xenrc params in your localrc to suite your taste
 source xenrc