XenServer: Add XenServer support

Upstreaming several changes from XenServer CI:
 - Use tty0 as console - thus the user can see what's happening during
   installation over VNC console.
 - Set the number of VCPUs to 6 for the DomU appliance
 - Add prepare_node_xenserver.sh script.
 - Use testing xva 1.1.4 which has trusty.

Change-Id: I56fc4e1af12f3f13fd1aa0b772bd8734b412d3d5
Merge "XenServer: Add XenServer support"