simple_xen_cookbook.rb

#
# Attribute file/per-node hash
xen_vms = Mash.new
xen_vms[:my_vm] = {
  :ip => '10.8.2.204', 
  :disk => '5Gb', 
  :memory => '2048Mb'
}

## 
# Recipe
##

node[:xen_vms].each do |vm|
 ... all your stuff, vm[:ip], vm[:disk], vm[:memory]
end