virttest.virsh: Add a parameter to support "virsh -c uri" session without autologin.

Hey, @cevich @yangdongsheng @liyangfnst

In virsh.VirshPersistent, When connecting to remote server from local with "virsh -c uri...",
if auto-login is not set, creating session will be impossible.
By importing a new parameter "transporter", flaging the connection type
(VirshConnectBack: remote to remote, VirshPersistent: local to remote).