security: aa-helper: allow virt-aa-helper to read /dev/dri

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>