security: aa-helper: allow virt-aa-helper to read /dev/dri
    
    Change fb01e1a44 "virt-aa-helper: generate rules for gl enabled
    graphics devices" implemented the detection for gl enabled
    devices in virt-aa-helper. But it will in certain cases e.g. if
    no rendernode was explicitly specified need to read /dev/dri
    which it currently isn't allowed.
    
    Add a rule to the apparmor profile of virt-aa-helper itself to
    be able to do that.
    
    Acked-by: Jamie Strandboge <jamie@canonical.com>
    Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>