virt-aa-helper: grant locking permission on -f
    
    Hot-adding disks does not parse the full XML to generate apparmor rules.
    Instead it uses -f <PATH> to append a generic rule for that file path.
    
    580cdaa7: "virt-aa-helper: locking disk files for qemu 2.10" implemented
    the qemu 2.10 requirement to allow locking on disks images that are part of
    the domain xml.
    
    But on attach-device a user will still trigger an apparmor deny by going
    through virt-aa-helper -f, to fix that add the lock "k" permission to the
    append file case of virt-aa-helper.
    
    Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>