Enable Xen/DevStackDomU to have larger disk

  * Size of xvda can be specified via xenrc

  * Fixes bug 1076430

Change-Id: Ia4ffef98b01fa9572e43c46275a132b2b1e5f689
diff --git a/tools/xen/xenrc b/tools/xen/xenrc
index 0365a25..1a5a2a9 100644
--- a/tools/xen/xenrc
+++ b/tools/xen/xenrc
@@ -11,6 +11,7 @@
 # Size of image
 VDI_MB=${VDI_MB:-5000}
 OSDOMU_MEM_MB=1024
+OSDOMU_VDI_GB=8
 
 # VM Password
 GUEST_PASSWORD=${GUEST_PASSWORD:-secrete}