Make devstack work with xcp-xapi package on Ubuntu 12.04

- allow you to configure the xenapi_user (often other than root)
- allow you to disable the guest installer network
- install the plugins in the xcp-xapi location
- use alternate webserver location when adding the preseed file
- skip the centos specific ip forwarding configuration
- make use xcp inventory, if no xensource-inventory is found
- correctly deal with kpartx to mount the VM VDI in manage_vdi

Change-Id: I8d51725fc97f0bcaa27a46f7a7ced13c369c809e
diff --git a/tools/xen/xenrc b/tools/xen/xenrc
index bf3d16a..102a492 100644
--- a/tools/xen/xenrc
+++ b/tools/xen/xenrc
@@ -38,5 +38,8 @@
 MGT_VLAN=${MGT_VLAN:-101}
 MGT_DEV=${MGT_DEV:-eth0}
 
+# Guest installer network
+ENABLE_GI=true
+
 # Source params
 cd ../.. && source ./stackrc && cd $TOP_DIR