Some tweaks for xen + devstack.

 * Import functions for git_clone (allows RECLONE for xen plugins)
 * Fix a potential xvas path issue
 * Tweaks to docs

Change-Id: I34f5c57a53884dfe944f3b0eb8896c57e348e389
diff --git a/tools/xen/xenrc b/tools/xen/xenrc
index 73f9c02..58fda31 100644
--- a/tools/xen/xenrc
+++ b/tools/xen/xenrc
@@ -36,7 +36,7 @@
 MGT_DEV=${MGT_DEV:-eth0}
 
 # XVA Directory
-XVA_DIR=${XVA_DIR:-xvas}
+XVA_DIR=${XVA_DIR:-`pwd`/xvas}
 
 # Path to xva file
 XVA=${XVA:-$XVA_DIR/$GUEST_NAME.xva }