session-info: check for a locked session
Each session back-end can return this information to vdagentd when
requested.
The agent should use this on situations that should not work when
session is locked such as file-transfer-start which is fixed by this
patch.
systemd-login is the only back-end implementing this function at the
moment and I'll address console-kit back-end in a later patch.
Also, this patch makes spice-vdagent depend on dbus for getting the
lock information.
Resolve: https://bugzilla.redhat.com/show_bug.cgi?id=
1323623
Acked-by: Jonathon Jongsma <jjongsma@redhat.com>