session-info back-ends such as console-kit and systemd-login have the
concept of session's class which informs if session belongs to user or
not [0]. We can disable features based on the session class.

[0] Display-Managers are 'Greeter' and Display lock screens are
'lock-screen'

This patch introduces session_info_is_user() and disable file-xfer in
case agent's session does not belong to the 'user' class. As the
session-info data is hold by vdagentd, this patch also introduces
VDAGENTD_FILE_XFER_DISABLE message to disable file-xfer that is done
in vdagent.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1328761



Acked-by: Jonathon Jongsma <jjongsma@redhat.com>