CA-140330: Use originator in session.login_with_password

-Calling new overload of the session.login_with_password() XAPI function (+originator field). This lets XenServer to group all sessions from a given XenCenter instance.
-Refactoring Session.cs to hide API (overload) differences. I have kept only one method for login in this class that will do the login as it used to, but also passing the "originator" field to the API function. This is set to (static string) Session.UserAgent that is unique for all XC instances.

Signed-off-by: Gabor Apati-Nagy <gabor.apati-nagy@citrix.com>