Add our own cookie jar to librest soup sessions

This will allow us to inject a JSESSIONID cookie if the user specifies one.
Add OvirtProxy::jsessionid property

This will be useful to get persistent authentication to an oVirt session
Use persistent authentication

Now that we can handle cookies in the HTTP session with the oVirt engine,
we can set the Prefer: persistent-auth header so that the user is not
being asked for a password for every REST request.