[Fix] Doc

Issue: - It is not possible to pass [R]andom [N]umber [G]enerator
         device to libvirt guest xml in order to control entropy.

       - Doc has no information on how to provision vms using salt

Solution: - Pass rng parameters via kwargs from node: pillar
            Attach rng xml object to generated xml.

          - Provide with an example

Prod-Related: PROD-19214
Customer-Found
Change-Id: Iea111f2d927edf46f06bb7ccfad06d37b752fba9

Change-Id: I685de4207f1f7f17264cc4ad2d81c3348ca20074

Example:

parameters:
  salt:
    control:
      size: #RAM 4096,8192,16384,32768,65536
        openstack.control:
          cpu: 8
          ram: 32768
          disk_profile: small_two_disks
          net_profile: default

  virt:
    disk:
      small_two_disks:
        - system:
            size: 50000
            image: http:/path/to/image.qcow2
        - cinder-volume:
            size: 50000

Change-Id: Ie092b77627cb2b1d36ba4b7a2a7d749ad91881c8