def exists?(vm_name = nil)
      return (vm_names.count > 0) if vm_name.nil?
      vm_names.include? vm_name.to_sym
    end
def exists?(vm_name = nil)
      return (vm_names.count > 0) if vm_name.nil?
      vm_names.include? vm_name.to_sym
    end
Determines if a particular virtual machine exists in the output

   * vm_name: The name of the virtual machine to look for