virt-libvirt: Add create and "destroy" methods

* Added a create method which mostly wrapps virsh
  net-create command to adds and starts a _transient_
  (NOT persistent) host network.

* Added the compliment of new_all_networks_dict() which
  utterly and completely removes ALL defined host
  networks unconditionally.  The method name guards
  against foolish usage (You made a backup right?).
Signed-off-by: NChris Evich <cevich@redhat.com>