Thing to add the missing method "create_snapshot" to the libvirt ruby domain bindings. Both the libvirt and builder libraries are in debian packages (apt-get install ruby-{libvirt,builder})

libvirtsnapshot.rb

require 'libvirt'
require 'builder'

class Libvirt::Domain
  def create_snapshot(sdesc=nil)
  	builder = Builder::XmlMarkup.new
  	snapshot_xml = builder.domainsnapshot do |ds|
  	  if sdesc
  	  	ds.description(sdesc)
	  end
	end
	self.snapshot_create_xml(snapshot_xml)
  end
end

c = Libvirt::open('qemu:///system')
d = c.lookup_domain_by_name 'jeff'
d.create_snapshot("Testing123")
p d.list_snapshots